cel_manager: Don't crash if configuration file is invalid.

The cel_manager module did not properly handle the case where the
configuration file was invalid. The module will now output a warning
message and disable itself if this occurs.

Reported by: Bryan Walters


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@405581 f38db490-d61c-443f-a65b-d21fe96a405b