Provide more detailed error message in case of version mismatch

this is the most likely cause of redirect loops when using docker,
so record the spawner version and check it when a redirect is detected.

In the event of a redirect and mismatch, fail with a message explaining the version mismatch and how to fix it.

jupyterhub/user.py

@@ -472,6 +472,9 @@ class User(HasTraits):
         else:
             server_version = resp.headers.get('X-JupyterHub-Version')
             _check_version(__version__, server_version, self.log)
+            # record the Spawner version for better error messages
+            # if it doesn't work
+            spawner._jupyterhub_version = server_version
         finally:
             spawner._waiting_for_response = False
             spawner._start_pending = False