res/ari/resource_channels: Don't return allocation failure on failed function

If a function fails to execute, it is most likely due to one of two reasons:
(1) The function doesn't exist or can't be read from
(2) The function is dangerous and is restricted based on the user's permissions

Currently we return allocation failure, which is incorrect. This updates the
reason code to more accurately reflect why the request failed.

ASTERISK-24215


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@421311 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/ari/resource_channels.c

@@ -964,7 +964,8 @@ void ast_ari_channels_get_channel_var(struct ast_variable *headers,
 
 	if (args->variable[strlen(args->variable) - 1] == ')') {
 		if (ast_func_read2(channel, args->variable, &value, 0)) {
-			ast_ari_response_alloc_failed(response);
+			ast_ari_response_error(
+				response, 500, "Unable to read provided function");
 			return;
 		}
 	} else {