res_ari: Cannot get control also means channel is unavailable.

The only caller of ari_bridges_play_found() has this note:

If ari_bridges_play_found fails because the channel is unavailable for
playback, The channel will be removed from the playback list soon.  We can
keep trying to get channels from the list until we either get one that
will work or else there isn't a channel for this bridge anymore, in which
case we'll revert to ari_bridges_play_new.

Change-Id: Ib068141b367ccaa17be0dab4181c98e26c5127d6

res/ari/resource_bridges.c

@@ -526,9 +526,7 @@ static enum play_found_result ari_bridges_play_found(const char *args_media,
 
 	control = stasis_app_control_find_by_channel(play_channel);
 	if (!control) {
-		ast_ari_response_error(
-			response, 500, "Internal Error", "Failed to get control snapshot");
-		return PLAY_FOUND_FAILURE;
+		return PLAY_FOUND_CHANNEL_UNAVAILABLE;
 	}
 
 	ao2_lock(control);