Merge branch 'kerberos-failure-message' into 'master'

Add hints for Kerberos sign-in failure

We do not know what went wrong when this page is shown but we can guess.

See merge request !583

app/views/errors/kerberos_denied.html.haml

@@ -2,6 +2,15 @@
 %h1 401
 %h3 Kerberos authentication failed
 %hr
+%p
+  Possible causes:
+  %ul
+    %li
+      You do not have a local Ticket Granting Ticket
+    %li
+      Your browser does not support Kerberos
+    %li
+      You do not have access to this service
 %p
   Return to the
   = succeed "." do