Adding some generic handling of error codes sent to us in replys to requests.

Previously they always set hangupcause 0, which is generally wrong. With this
change, we're setting some generic hangup causes. For 5xx errors, which indicate
some sort of problem with the remote server, we're now setting CONGESTION.

EDVX002


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@197266 65c4cc65-6c06-0410-ace0-fbb531ad65f3