This sets a timeout on the nbexec call to the JupyterLab server
extension for notebook execution. It raises a NbexecTaskTimeout
exception in the arq worker, which should result in an errored state for
the notebook execution job. We might need to fine tune the exception
handling so that the result output in the API is useful.

This modifies the result payload for a notebook execution job when the
job is complete.

- Add a new "error" field that reports an exception raised during the
  noteburst job, which explains why a job's "success" field is false.
  The field contains a NoteburstExecutionError model, which in turn
  contains an enum code for the error and a text message describing the
  error.

- Keep the existing "ipynb_error" field for reporting exceptions raised
  within the notebook itself. Noteburst's execution could still be
  considered successful even if the notebook itself raised an exception.

This will let a user know what the timeout setting was for the job,
which is especially helpful if the execution was related to the timeout.