Previosuly, 404 errors showed a generic error message, making it unclear
what could have gone wrong. For security reasons, we don't want people
to know if the group does not exist or if the group exists but the user
does not have permission. This change updates the error message to
reflect both of these possibilities.
Bug: Issue 8252
Change-Id: I6940d53b342a81ca10ec78977de4ec548fd10a6f