Do not raise exception for pipenv graph when virtualenv does not exist

[jxltom]

Issue description

When virutaulenv is not created for a project yet, pipenv graph will raise exception.

➜  pipenv graph
Traceback (most recent call last):
  File "/usr/local/bin/pipenv", line 11, in <module>
    load_entry_point('pipenv', 'console_scripts', 'pipenv')()
  File "/home/user/Documents/dev/pipenv/pipenv/vendor/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/home/user/Documents/dev/pipenv/pipenv/vendor/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/home/user/Documents/dev/pipenv/pipenv/vendor/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/user/Documents/dev/pipenv/pipenv/vendor/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/user/Documents/dev/pipenv/pipenv/vendor/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/user/Documents/dev/pipenv/pipenv/cli/command.py", line 538, in graph
    do_graph(bare=bare, json=json, json_tree=json_tree, reverse=reverse)
  File "/home/user/Documents/dev/pipenv/pipenv/core.py", line 2430, in do_graph
    python_path = which("python")
  File "/home/user/Documents/dev/pipenv/pipenv/core.py", line 110, in which
    raise RuntimeError("location not created nor specified")
RuntimeError: location not created nor specified

It happens at least for v2018.10.13.

Expected result

It should be a friendly info.

➜  pipenv graph
Warning: No virtualenv has been created for this project yet! Consider running `pipenv install` first to automatically generate one for you or see`pipenv install --help` for further instructions.

---

Please run $ pipenv --support, and paste the results here. Don't put backticks (`) around it! The output already contains Markdown formatting.

If you're on macOS, run the following:

$ pipenv --support | pbcopy

If you're on Windows, run the following:

> pipenv --support | clip

If you're on Linux, run the following:

$ pipenv --support | xclip

[DannyDannyDanny]

I'm having an issue with setting up Pipenv in a certain directory. There used to be an old pipenv there, which I tried to delete:

• deleted env with pew rm [env-name]
• manually deleted the Pipfile from the directory (it was not deleted by pew rm)
• tried to setup a new pipenv with pipenv shell

Last step failed with following output:

Creating a Pipfile for this project…
Traceback (most recent call last):
  File "/usr/local/bin/pipenv", line 11, in <module>
    sys.exit(cli())
  File "/usr/local/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/pipenv/vendor/click/decorators.py", line 64, in new_func
    return ctx.invoke(f, obj, *args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/pipenv/cli/command.py", line 374, in shell
    pypi_mirror=state.pypi_mirror,
  File "/usr/local/lib/python3.7/site-packages/pipenv/core.py", line 2069, in do_shell
    ensure_project(three=three, python=python, validate=False, pypi_mirror=pypi_mirror)
  File "/usr/local/lib/python3.7/site-packages/pipenv/core.py", line 601, in ensure_project
    validate=validate, skip_requirements=skip_requirements, system=system
  File "/usr/local/lib/python3.7/site-packages/pipenv/core.py", line 324, in ensure_pipfile
    project.create_pipfile(python=python)
  File "/usr/local/lib/python3.7/site-packages/pipenv/project.py", line 673, in create_pipfile
    required_python = self.which("python", self.virtualenv_location)
  File "/usr/local/lib/python3.7/site-packages/pipenv/core.py", line 114, in which
    raise RuntimeError("location not created nor specified")
RuntimeError: location not created nor specified

pbcopy from pipenv --support | pbcopy:

<details><summary>$ pipenv --support</summary>

Pipenv version: `'2018.10.13'`

Pipenv location: `'/usr/local/lib/python3.7/site-packages/pipenv'`

Python location: `'/usr/local/opt/python/bin/python3.7'`

Python installations found:

pipenv --support | pbcopy also crashes with following output:

Courtesy Notice: Pipenv found itself running within a virtual environment, so it will automatically use that environment, instead of creating its own for any project. You can set PIPENV_IGNORE_VIRTUALENVS=1 to force pipenv to ignore that environment and create its own instead. You can set PIPENV_VERBOSITY=-1 to suppress this warning.
Traceback (most recent call last):
  File "/usr/local/bin/pipenv", line 11, in <module>
    sys.exit(cli())
  File "/usr/local/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 1114, in invoke
    return Command.invoke(self, ctx)
  File "/usr/local/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/pipenv/vendor/click/decorators.py", line 64, in new_func
    return ctx.invoke(f, obj, *args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/pipenv/vendor/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/pipenv/cli/command.py", line 142, in cli
    get_pipenv_diagnostics()
  File "/usr/local/lib/python3.7/site-packages/pipenv/help.py", line 33, in get_pipenv_diagnostics
    python_paths = finder.find_all_python_versions()
  File "/usr/local/lib/python3.7/site-packages/pipenv/vendor/pythonfinder/pythonfinder.py", line 112, in find_all_python_versions
    major=major, minor=minor, patch=patch, pre=pre, dev=dev, arch=arch
  File "/usr/local/lib/python3.7/site-packages/pipenv/vendor/pythonfinder/models/path.py", line 222, in find_all_python_versions
    return [c for c in sorted(path_filter, key=version_sort, reverse=True)]
AttributeError: 'list' object has no attribute 'as_python'

I'm going to try delete the whole directory, make a new directory (with same name) and try again.