webadmin: Wrong error on decreasing data center compatibility version

A wrong error message is given when trying to decrease data center
compatibility version. When editing a data center to change its
compatibility version the UI shows error message "Cannot edit
Repository. Compatibility version is invalid, previous value restored."

This patch changes the error message to "Cannot decrease data center
compatibility version."

Change-Id: Ic55a2c180e0e5b86477afaacc4e1daf65fb2c570
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=904729
Signed-off-by: Ravi Nori <rnori@redhat.com>