Correct unresolved ugettext_lazy error strings in git_export command

**Background**: Error messages like `Error: <django.utils.functional.__proxy__ object at 0x79a8710>` aren't terribly helpful.  This should resolve those strings to the correct error message.

**Studio**: Small command bug fix

**LMS**: None

**Testing**: running `python manage.py cms --settings=aws git_export silly foo/bar:baz` should produce the error message: `Error: Bad course location provided` instead of `Error: <django.utils.functional.__proxy__ object at ...`