engine : CommandExecutor should handle exception in CallBackMethods

If one of the callback methods raises an exception,
the framework keeps calling the method again the next
loop iteration.  In some circumstances this means the
command will never converge--e.g. coding errors,
database connection failures, etc.

Change-Id: I06b82f29ac31fd09903d76f87e839d8ac32ef1e1
Bug-Url: https://bugzilla.redhat.com/1121237
Signed-off-by: Ravi Nori <rnori@redhat.com>