stasis: set a channel variable on websocket disconnect error

Resolve compile errors caused by r433863 by fixing the
documentation xml to comply with the schema.
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Merged revisions 433888 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@433891 65c4cc65-6c06-0410-ace0-fbb531ad65f3