Stop delete force failing when virtualenv does not exist

[eganjs]

When pyenv virtualenv-delete -f non-existent-virtualenv is run then an error message is displayed and the return code is 1

This is contrary to the behavior described by pyenv virtualenv-delete --help:

Usage: pyenv virtualenv-delete [-f|--force] <virtualenv>

   -f  Attempt to remove the specified virtualenv without prompting
       for confirmation. If the virtualenv does not exist, do not
       display an error message.

See `pyenv virtualenvs` for a complete list of installed versions.

Specifically for the force flag: If the virtualenv does not exist, do not display an error message.

This PR addresses this by checking that the force flag is not set before continuing with standard behavior (displaying the error message and returning the non-zero code).