Show error messages on spawn form

when spawning fails

instead of 500

jupyterhub/handlers/pages.py

@@ -54,6 +54,14 @@ class SpawnHandler(BaseHandler):
     
     Only enabled when Spawner.options_form is defined.
     """
+    def _render_form(self, message=''):
+        user = self.get_current_user()
+        return self.render_template('spawn.html',
+            user=user,
+            spawner_options_form=user.spawner.options_form,
+            error_message=message,
+        )
+
     @web.authenticated
     def get(self):
         """GET renders form for spawning with user-specified options"""
@@ -64,11 +72,7 @@ class SpawnHandler(BaseHandler):
             self.redirect(url)
             return
         if user.spawner.options_form:
-            html = self.render_template('spawn.html',
+            self.finish(self._render_form())
-                user=self.get_current_user(),
-                spawner_options_form=user.spawner.options_form,
-            )
-            self.finish(html)
         else:
             # not running, no form. Trigger spawn.
             url = url_path_join(self.base_url, 'user', user.name)
@@ -90,7 +94,12 @@ class SpawnHandler(BaseHandler):
         for key, byte_list in self.request.files.items():
             form_options["%s_file"%key] = byte_list
         options = user.spawner.options_from_form(form_options)
-        yield self.spawn_single_user(user, options=options)
+        try:
+            yield self.spawn_single_user(user, options=options)
+        except Exception as e:
+            self.log.error("Failed to spawn single-user server with form", exc_info=True)
+            self.finish(self._render_form(str(e)))
+            return
         self.set_login_cookie(user)
         url = user.server.base_url
         self.redirect(url)

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>