`ghq` does not work well with multiple ghq.root with nonexistent directory

[x-motemen]

`ghq` does not work well with multiple ghq.root with nonexistent directory

[znz]

Thank you for your explanation.
I understand changing behavior.
So I create the directory or unset nonexistent directory from ghq.root.

[Songmu]

This behavior changed after #60 is merged.

I made this change because I thought it would be better to raise the error clearer than to ignore if the directory doesn't exist.

However, it may be better to create automatically when we ghq get. Even in the case of ghq list, it is possible to specify a relative path such as ./vendor in GHQ_ROOT, so it may be better not to raise an error after listing the primary root.