correct handling of get_destination return values

correct handling of get_destination return values

A failure when calling the get_destination can mean multiple things.  If
the extension is not found, a 404 error is appropriate, but if the URI
scheme is incorrect, a 404 is not approperiate.  This patch adds the
get_destination_result enum to differentiate between these and other failure
types.  The only logical difference in this patch is that we now send a "416
Unsupported URI scheme" response instead of a "404" when the scheme is not
recognized.  This indicates to the initiator of the INVITE to retry the request
with a correct URI.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@273427 65c4cc65-6c06-0410-ace0-fbb531ad65f3