Handle exceptions in the load_session

[weekenthralling]

When refreshing the kernel, there might be other exceptions causing the refresh to fail. Perhaps catching exceptions would be more elegant.

For example:
When querying a non-existent kernel, the load_session method from the webhook returns a 404 error. If this is not handled, the final API response is a 500 error, whereas the expected response is a 404.

[kevin-bates]

LGTM - thank you @weekenthralling.