sway/output: Improve logging of swaybg execvp failure and more checks

This doesn't catch the error if a background changing command is
executed via swaymsg, but improves logging.
The additional checks at least propagate if e.g. forking failed.

sway/commands/output.c

@@ -111,7 +111,10 @@ struct cmd_results *cmd_output(int argc, char **argv) {
 	if (!config->reloading && !config->validating) {
 		apply_output_config_to_outputs(output);
 		if (background) {
-			spawn_swaybg();
+			if (!spawn_swaybg()) {
+				return cmd_results_new(CMD_FAILURE,
+					"Failed to apply background configuration");
+			}
 		}
 	}