cmd: caddy fmt return code is 1 if not formatted (#5297)

* cmd: Fix caddy fmt if input isn't formatted

* Fixes #5294
* return exit 1 with an error message

* cmd: Use formattingDifference for caddy fmt

* #5294
* expose caddyfile.formattingDifference

cmd/commandfuncs.go

@@ -576,6 +576,10 @@ func cmdFmt(fl Flags) (int, error) {
 		fmt.Print(string(output))
 	}
 
+	if warning, diff := caddyfile.FormattingDifference(formatCmdConfigFile, input); diff {
+		return caddy.ExitCodeFailedStartup, fmt.Errorf("%s:%d: Caddyfile input is not formatted", warning.File, warning.Line)
+	}
+
 	return caddy.ExitCodeSuccess, nil
 }