Show error toast when file size exceeds the limits (#27985)

As title.
Before that, there was no alert at all.
After:

![error_toast](https://github.com/go-gitea/gitea/assets/70063547/c54ffeed-76f8-4c3a-b5dc-b9b3e0f8fc76)

web_src/js/features/common-global.js

@@ -247,6 +247,10 @@ export function initGlobalDropzone() {
  });
  }
  });
+ this.on('error', function (file, message) {
+ showErrorToast(message);
+ this.removeFile(file);
+ });
  },
  });
  }