[API] on 500 error only show message if gitea in dev mode (#11641) (#11753)

* add API specific InternalServerError()

* return 500 error msg only if not Production mode

* rm unnessesary change

modules/context/api.go

 
 	if status == http.StatusInternalServerError {
 		log.ErrorWithSkip(1, "%s: %s", title, message)
+
+		if macaron.Env == macaron.PROD {
+			message = ""
+		}
 	}
 
 	ctx.JSON(status, APIError{