Monitor: Audit handler return

This commit verifies the following two rules specified by
Markus Armbruster:

1. If the handler returns failure, it must have passed an error.

   If it didn't, it's broken. Report an internal error to the client,
   and report the bug to the programmer.

2. If the handler returns success, it must not have passed an error.

   If it did, it's broken. Report the error to the client, and report
   the bug to the programmer.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
