throw exception if plugin not found

if a plugin is not found, throw an exception rather than ignoring it

Change-Id: Iffa60c3a1e900465f41dc003263cd1e4d235ee9e
Bug-Url: https://bugzilla.redhat.com/1002443
Signed-off-by: Laszlo Hornyak <lhornyak@redhat.com>