Opaque error if registrant has no access to create Git repository

[Colin Watson]

If _BaseGitNamespace.createRepository finds that self.getDefaultInformationType(registrant) returns None, which happens if the registrant doesn't have full access to the necessary information type in the project in which they're trying to create the repository, then it raises GitRepositoryCreationForbidden() with no detailed error message. We should include an actual error message here, because otherwise it's completely opaque what's going on.