core: improve error msg when decrease DC version

Improve error message displayed to user when
an attempt is made to decrease data center version
(which is a not supported action)

Change-Id: I6c9eef9bfef0bd732d5ad09a549fbfda3666280f
Bug-Url: https://bugzilla.redhat.com/843914
Signed-off-by: Alissa Bonas <abonas@redhat.com>