Handle 'pyenv version-name' failure in pyenv_virtualenvwrapper_load

Before:

    % pyenv version
    pyenv: version `2.7.9rc1' is not installed
    (eval):source:80: no such file or directory:
    pyenv: version `2.7.9rc1' is not installed

After:

    % 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
Merge pull request #20 from blueyed/handle-version-name-failure

Handle 'pyenv version-name' failure in pyenv_virtualenvwrapper_load