stasis: set a channel variable on websocket disconnect error

When an error occurs while writing to a web socket, the web socket is
disconnected and the event is logged. A side-effect of this, however, is that
any application on the other side waiting for a response from Stasis is left
hanging indefinitely (as there is no mechanism presently available for
notifying interested parties about web socket error states in Stasis).

To remedy this scenario, this patch introduces a new channel variable:
STASISSTATUS.

The possible values for STASISSTATUS are:
SUCCESS         - The channel has exited Stasis without any failures
FAILED          - Something caused Stasis to croak. Some (not all) possible
                  reasons for this:
                    - The app registry is not instantiated;
                    - The app requested is not registered;
                    - The app requested is not active;
                    - Stasis couldn't send a start message

ASTERISK-24802
Reported By: Kevin Harwell
Review: https://reviewboard.asterisk.org/r/4519/
........

Merged revisions 433839 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@433845 65c4cc65-6c06-0410-ace0-fbb531ad65f3