sessionmanager KeyError when restarting kernel

One of our internal users received the following traceback when restarting a metakernel based kernel via the Kernel -> Restart menu item. The traceback appeared in a popup dialog over the notebook page.

Notebook 5.1.0 final running in Python 3.6.

```
Traceback (most recent call last):
  File "/opt/jupyterhub/envs/notebook-1506165706/lib/python3.6/site-packages/notebook/base/handlers.py", line 540, in wrapper
    result = yield gen.maybe_future(method(self, *args, **kwargs))
  File "/opt/jupyterhub/envs/notebook-1506165706/lib/python3.6/site-packages/tornado/gen.py", line 1055, in run
    value = future.result()
  File "/opt/jupyterhub/envs/notebook-1506165706/lib/python3.6/site-packages/tornado/concurrent.py", line 238, in result
    raise_exc_info(self._exc_info)
  File "", line 4, in raise_exc_info
  File "/opt/jupyterhub/envs/notebook-1506165706/lib/python3.6/site-packages/tornado/gen.py", line 1069, in run
    yielded = self.gen.send(value)
  File "/opt/jupyterhub/envs/notebook-1506165706/lib/python3.6/site-packages/notebook/services/sessions/handlers.py", line 69, in post
    model = yield gen.maybe_future(sm.get_session(path=path))
  File "/opt/jupyterhub/envs/notebook-1506165706/lib/python3.6/site-packages/notebook/services/sessions/sessionmanager.py", line 170, in get_session
    return self.row_to_model(row)
  File "/opt/jupyterhub/envs/notebook-1506165706/lib/python3.6/site-packages/notebook/services/sessions/sessionmanager.py", line 209, in row_to_model
    raise KeyError
KeyError
```

Start with notebook/services/sessions/sessionmanager.py, especially get_session and row_to_model, then review the request path in notebook/services/sessions/handlers.py. Reproduce the failure by restarting a metakernel-based kernel from the Kernel → Restart menu; done means the restart no longer produces the reported KeyError popup.

Written by the indexing model from the issue text.