Improve error code when using tfenv list before a version has been selected for the first time

[jon-flowers]

Hello,
I found after first installing this that you can get into an error scenario early on. The tfenv list command is meant to show all available local versions. However, if you follow a certain path in the initial setup, this command will only give you an error, one which may not be as clear as it should be.

Steps to Reproduce:

1. Install tfenv
2. tfenv install <any version>
3. Terraform version installs as expected
4. tfenv list
5. You receive an error: "cat: /usr/local/Cellar/tfenv/2.0.0/version: No such file or directory
   Version could not be resolved (set by /usr/local/Cellar/tfenv/2.0.0/version or tfenv use )
   tfenv-version-name failed"

This is obviously happening because the version file has not been created yet, because I have not entered tfenv use <version> for the first time after installing my first TF version. However, while the terminal log does say To make this your default version, run 'tfenv use <version>' after installing a new version, it's not clear that you have to do this at least once before you can even list what versions you have.

While this is a relatively "beginner" error, the wording of the errors did not immediately point me to the error. Instead, I initially assumed my tfenv package was corrupted and so re-installed it.

If entering tfenv use <version> at least once is required before the tfenv list command can be used at all, then it may at least be helpful to make either the error messages or the initial steps in the documentation clearer so that others aren't confused by this in the future.

Thanks!