[Documentation] Write error to file for debugging

[jan-janssen]

At the moment the exception is just transferred, but there is not option to write the exception to a file, this would be a great extension:

executorlib/executorlib/task_scheduler/interactive/fluxspawner.py

Line 116 in f6e96a2

if self._flux_log_files and self._cwd is not None:

[jan-janssen]

This functionality was implemented in #736

It is now possible to set the error file in the resource directory, by setting:

"error_log_file": "error.out"

[jan-janssen]

The documentation for this functionality was added in #751