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");
+			}
 		}
 	}
 

sway/config/output.c

@@ -822,7 +822,9 @@ static bool _spawn_swaybg(char **command) {
 			setenv("WAYLAND_SOCKET", wayland_socket_str, true);
 
 			execvp(command[0], command);
-			sway_log_errno(SWAY_ERROR, "execvp failed");
+			sway_log_errno(SWAY_ERROR, "failed to execute '%s' "
+				"(background configuration probably not applied)",
+				command[0]);
 			_exit(EXIT_FAILURE);
 		}
 		_exit(EXIT_SUCCESS);
@@ -832,12 +834,13 @@ static bool _spawn_swaybg(char **command) {
 		sway_log_errno(SWAY_ERROR, "close failed");
 		return false;
 	}
-	if (waitpid(pid, NULL, 0) < 0) {
+	int fork_status = 0;
+	if (waitpid(pid, &fork_status, 0) < 0) {
 		sway_log_errno(SWAY_ERROR, "waitpid failed");
 		return false;
 	}
 
-	return true;
+	return WIFEXITED(fork_status) && WEXITSTATUS(fork_status) == EXIT_SUCCESS;
 }
 
 bool spawn_swaybg(void) {