Make sure `nvm ls foo` returns a nonzero exit code when a version is not found.

Fixes #440.
Make sure these vars are local, where supported
Don't bother trying to return the exit code of `nvm alias` or `nvm_print_versions`
Merge pull request #444 from creationix/fix_nvm_ls

Make sure `nvm ls foo` returns a nonzero exit code when a version is not found