Adding a note to UPGRADE.txt about a change made to res_agi in order to

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)
........

Merged revisions 396196 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@396197 65c4cc65-6c06-0410-ace0-fbb531ad65f3