Show error messages on spawn form

when spawning fails

instead of 500

share/jupyter/hub/templates/spawn.html

@@ -7,6 +7,11 @@
     <h1>Spawner options</h1>
   </div>
   <div class="row col-sm-offset-2 col-sm-8">
+      {% if error_message %}
+        <p class="spawn-error-msg text-danger">
+          Error: {{error_message}}
+        </p>
+      {% endif %}
     <form enctype="multipart/form-data" id="spawn_form" action="{{base_url}}spawn" method="post" role="form">
       {{spawner_options_form}}
       <br>