Generate error message when AMI action originate extension doesn't exist

Review: https://reviewboard.asterisk.org/r/1445/

Is this a bug or a new feature? No responses on Asterisk-dev so I'm 
committing to trunk only.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@339206 65c4cc65-6c06-0410-ace0-fbb531ad65f3