Fix some HTTP status codes in sessions API

IPython/html/services/sessions/handlers.py

@@ -63,6 +63,7 b' class SessionRootHandler(IPythonHandler):'
             kernel_id = km.start_kernel(cwd=nbm.notebook_dir)
             model = sm.create_session(name=name, path=path, kernel_id=kernel_id, ws_url=self.ws_url)
         self.set_header('Location', '{0}/api/sessions/{1}'.format(self.base_project_url, model['id']))
+        self.set_status(201)
         self.finish(json.dumps(model, default=date_default))
 
 class SessionHandler(IPythonHandler):

IPython/html/services/sessions/sessionmanager.py

@@ -133,7 +133,7 b' class SessionManager(LoggingConfigurable):'
         if reply is not None:
             model = self.reply_to_dictionary_model(reply)
         else:
-            model = None
+            raise web.HTTPError(404, u'Session not found: %s=%r' % (column, value))
         return model
 
     def update_session(self, session_id, **kwargs):
@@ -151,8 +151,6 b' class SessionManager(LoggingConfigurable):'
             and the value replaces the current value in the session 
             with session_id.
         """
-        column = kwargs.keys() # uses only the first kwarg that is entered
-        value = kwargs.values()
         for kwarg in kwargs:
             try:
                 self.cursor.execute("UPDATE session SET %s=? WHERE id=?" %kwarg, (kwargs[kwarg], session_id))