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
........

Merged revisions 405581 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@405582 65c4cc65-6c06-0410-ace0-fbb531ad65f3