packaging: setup: Making error messages more verbose

Making error messages a bit more verbose to help the user
pointing out possible problems.

Change-Id: I0a722d37420f4a3c9b0af09dc483cc9d73b4faff
Bug-Url: https://bugzilla.redhat.com/1144836
Signed-off-by: Simone Tiraboschi <stirabos@redhat.com>