engine: show the errors from external scheduler in the event log

If there is a problem with the external scheduler or it's plugins,
the user should be notified about that. This patch adds support for
the new scheduler xmlrpc api which contains the information about,
errors encountered while running the plugins.

Change-Id: Idd4dbe269d2885a6be04b82a3e2320c4f96c2387
Bug-Url: https://bugzilla.redhat.com/1056540
Signed-off-by: Jiri Moskovcak <jmoskovc@redhat.com>