Fix: GitLab UI 404 page style error suing Chinese characters

Add `flex: 1` for `div.field` element to make sure button won't gets squeezed to break line.
Changelog: fixed

app/assets/stylesheets/errors.scss

@@ -91,6 +91,7 @@ a {
     .field {
       margin-bottom: 0;
       margin-right: 0.5em;
+      flex: 1;
     }
   }
 }