Adding a note to UPGRADE.txt about a change made to res_agi in order to
indicate when streaming an audio file fails like it is done in other parts
of the code to indicate an error.

Note was requested by Paul Belanger: 
http://lists.digium.com/pipermail/asterisk-dev/2013-July/061420.html

(related to issue ASTERISK-21903)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@396196 f38db490-d61c-443f-a65b-d21fe96a405b