handle KeyError when get session

[ccw630]

If gateway is enabled, row_to_model can delete session row when kernel culled or died on gateway. Then it raises a KeyError unhadled in get_session.

[kevin-bates]

LGTM - thank you.

[ccw630]

Wondering if I should make a same PR to repo jupyter_server - what's the diffrence between those backend code?

[kevin-bates]

Yes, that would be helpful, thank you.
Jupyter server was derived directly from notebook but only includes backend code. That is, it is only the server portion.
The notebook repository is essentially in a maintenance-only mode while jupyter_server entertains feature additions, etc. Front-end applications have or are migrating to jupyter_server.