run_worker(), which establishes a Perforce session, used to take a

function as a parameter to run, which meant that callers had to define

an extra function or provide a lambda. Now it works as a context

manager, reducing the work needed by the callers.
As part of this, run_worker() now handles converting exceptions by

itself, without needing the old _convert_p4exception_to_scmexception()

utility method, simplifying code a bit more.
This also fleshes out the documentation for the affected functions.

Unit tests passed.
Tested Perforce functionality manually. Everything works as it did

before.