Cylc UI Server: document how to troubleshoot asyncio errors

[kinow]

Exceptions in the asyncio loop may not be logged/displayed to the user. We should document somewhere how to troubleshoot or identify these issues. I suspect it will go to Cylc Doc, somewhere a user/site admin would read.

See cylc/cylc-uiserver#65 (comment) comment for instructions how to enable it.

The official docs for Python 3 asyncio: https://docs.python.org/3/library/asyncio-dev.html#debug-mode

[kinow]

ps: should we have a label for each component? Or just the title prefix is fine?

[oliver-sanders]

The nature of this issue may change with cylc/cylc-uiserver#219

After which point it may become a slightly more generic Jupyter Server problem and we may be able to defer to the Jupyter documentation with instruction on how to redirect just the Cylc stuff into its own file if desirable.

[oliver-sanders]

Since cylc/cylc-uiserver#219 (and some other changes) things have now changed. Exceptions within the workflow scanning are logged and will soon be written to a log file by default.

This is now more a UIS issue to ensure that async code is configured correctly, if anything goes wrong we want a traceback or error message to be omitted.