filepath arg to generate_and_write_*_keypair may already exist

[lukpueh]

Description of issue or feature request:

As pointed out by @adityasaky in #169 (review) interface.generate_and_write_spx_keypair does not check if its filepath argument already exists. In the case of an existing directory we get an error (IOError on py2, IsADirectoryError on py3), in case of a file we silently override it. Moreover, since the function writes two files, i.e. <filepath> and <filepath>.pub, it might succeed writing one but not the other. The same applies to the rest of the generate_and_write_*_keypair functions in interface.

Current behavior:
filepath argument in interface.generate_and_write_*_keypair is not checked if it already exists

Expected behavior:
Check if <filepath> or <filepath>.pub already exist and act accordingly (unsurprisingly).

• @adityasaky suggests to warn and/or create the keys under <filepath>/<keyid> and <filepath>/<keyid>.pub. However, these could too already exist.

• I suggest to just fail if <filepath> or <filepath>.pub exist.
  OTOH, and given that there are many other reasons why the write operations might fail, we could just leave it as it is now, and maybe update the function docstrings (EAFP-style).