[core] Correct 500 error reporting

There is no site administrator email configuration exposed via Hue.
We could easily add it to settings.py's "ADMIN". For now, correct the
misleading error message.

desktop/core/src/desktop/templates/500.mako

@@ -20,8 +20,7 @@ ${commonheader("Error", "")}
 
   <div class="container-fluid">
     <h1>Server Error (500)</h1>
-    <p>Sorry, there's been an error. It's been reported to the site administrators
-    via e-mail and should be fixed shortly. Thanks for your patience.</p>
+    <p>Sorry, there's been an error. Please contact your site administrators. Thanks for your patience.</p>
   </div>
 
 ${commonfooter()}