tell the user about invalid/unknown commands before sending queued events

s->challenge does not need to be protected by the lock, since only process_message() (and functions it calls) ever use it, and process_message() is a blocking operation by definition


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@6703 65c4cc65-6c06-0410-ace0-fbb531ad65f3