Properly indicate failure to open an audio stream in res_agi

If there is an error streaming an audio file, the current return status makes it
difficult for an AGI script to determine that there was an error with the audio
file.

This patches changes the result to return -1 and the function returns
RESULT_FAILURE instead of RESULT_SUCCESS.  From looking at other parts of
res_agi, this would appear to be the proper way to handle an error.

(closes issue ASTERISK-21903)
Reported by: Ariel Wainer
Tested by: Ariel Wainer
Patches:
	asterisk-21903-return-stream-res_1.8.diff
					by Michael L. Young (license 5026)

Review: https://reviewboard.asterisk.org/r/2625/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@394640 f38db490-d61c-443f-a65b-d21fe96a405b