"pyenv version" with invalid versions should not fail

[blueyed]

% cat .pyenv/version
3.4.2
2.7.9rc1
2.7.8

% pyenv version
pyenv: version `2.7.9rc1' is not installed
pyenv-virtualenvwrapper: python is not available.
pyenv-virtualenvwrapper: failed to initialize virtualenvwrapper_lazy.
pyenv: version `2.7.9rc1' is not installed

I would like it to display the other versions, too.

[yyuu]

Thanks for good PR including tests 👍 Although, I don't think permissive approach is always good to deal with configuration errors.

When pyenv-version-name was invoked, there should be 3 cases.

1. All versions in pyenv-version-origin are available
2. Some versions in pyenv-version-origin are available
3. All versions in pyenv-version-origin are unavailable

In case 1 and 3, it's clear what to do. In case 2, pyenv v20141211 displays errors on standard error then exists with 1. This must be the most conservative behaviour.

OTOH, with this patch, it displays all available versions, then exits with 1 only if none is available. This is permissive behaviour than current v20141211.

I think being conservative is good enough since it indicates that there is some errors. The version in .python-version might be invalid to local pyenv installation since it might be managed by other users if the file is committed into the application repository. I think that the problem is in some of code in pyenv-virtualenv and pyenv-virtualenvwrapper which handles output from pyenv-version-name. They should be able to handle the errors from pyenv-version-name properly.