define.py: remove %s from imageErr

imageErr untypically has an %s in the error message.
To make things worse, most flows do not fill it, producing messages
which are difficult to understand.

This patch removes the aforementioned %s.

Change-Id: Ia2b8534c34e8efb9d4af3c438a0a0ecc99cf16f0
Bug-Url: https://bugzilla.redhat.com/960635
Signed-off-by: Allon Mureinik <amureini@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/18230
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>