diff --git a/IPython/html/services/sessions/handlers.py b/IPython/html/services/sessions/handlers.py
index addfd88..d88aa96 100644
--- a/IPython/html/services/sessions/handlers.py
+++ b/IPython/html/services/sessions/handlers.py
@@ -106,7 +106,11 @@ class SessionHandler(IPythonHandler):
     def delete(self, session_id):
         # Deletes the session with given session_id
         sm = self.session_manager
-        sm.delete_session(session_id)
+        try:
+            sm.delete_session(session_id)
+        except KeyError:
+            # the kernel was deleted but the session wasn't!
+            raise web.HTTPError(410, "Kernel deleted before session")
         self.set_status(204)
         self.finish()
 
diff --git a/IPython/html/static/base/js/utils.js b/IPython/html/static/base/js/utils.js
index aa85d17..cde25f3 100644
--- a/IPython/html/static/base/js/utils.js
+++ b/IPython/html/static/base/js/utils.js
@@ -523,12 +523,14 @@ define([
     var ajax_error_msg = function (jqXHR) {
         // Return a JSON error message if there is one,
         // otherwise the basic HTTP status text.
-        if (jqXHR.responseJSON && jqXHR.responseJSON.message) {
+        if (jqXHR.responseJSON && jqXHR.responseJSON.traceback) {
+            return jqXHR.responseJSON.traceback;
+        } else if (jqXHR.responseJSON && jqXHR.responseJSON.message) {
             return jqXHR.responseJSON.message;
         } else {
             return jqXHR.statusText;
         }
-    }
+    };
     var log_ajax_error = function (jqXHR, status, error) {
         // log ajax failures with informative messages
         var msg = "API request failed (" + jqXHR.status + "): ";