Better diagnostics and logging for failures of save_notebook

[davidwagner]

Is your feature request related to a problem? Please describe.
Currently the force-save implemented by grader.export(force_save=True) is flaky and we have observed it sometimes failing in Data 8. When it fails, the student sees the error message "AssertionError: Notebook project2.ipynb is empty. Please save and checkpoint your notebook and rerun this cell." We are having trouble identifying what is causing this failure.

Describe the solution you'd like
It would be useful to have more diagnostics about this. In particular:

1. It'd be useful to get this error into the syslog logs that are visible in Google Cloud Platform logs, so we can track how often it occurs and when it occurs. Right now the error is displayed to the student but it is not logged.
2. It would be useful to have additional logging about the cause of the problem. The relevant code calls IPython.display.display(IPython.display.Javascript("Jupyter.notebook.save_checkpoint();")), which is async, so any uncaught exceptions/errors will get swallowed. It would be nice to try to catch exceptions/errors and log them.

Additional context
#444,

otter-grader/otter/check/utils.py

Line 18 in 6a33a75

def save_notebook(filename, timeout=10):

[stale]

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