engine.py: return False on cert error screen

If we cannot download the engine certificate we should
only show the error screen and return.

Change-Id: I3e3f03365e77244c5521a11d92041371088b4362
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=895912
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/12244
Reviewed-by: Joey Boggs <jboggs@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>