fix: Log exception and exit

[giulioungaretti]

Closes #504

Changes proposed in this pull request:

• log exception in background task
• exit loop

@alan-geller , and @QCoDeS/core the question here is if we want to exit (early return) or not.
My feeling is that we may as well not exit, technically the data could be still good even if the background task failed, but one never knows.

The clean way is to return, but I am curious about your 💎 feedback !

[alan-geller]

The code is of course fine.

The original use for the background task was to update a live plot. In this case, I don't think the task throwing an exception should cause the loop to exit.

Other background tasks, of course, might properly cause the loop to fail if the task fails (throws).

I think it's reasonable to always fail if the background task throws, since you can always wrap the "real" background task in a try/catch that just ignores all exceptions (presumably other than keyboard interrupt). We should make sure that the docs call out this behavior.

As an aside, I would vote (strongly!) against adding another optional argument that flagged whether or not the loop should exit on a background task exception... There are far too many options already.

[alexcjohnson]

The logging is a great idea. I agree with @alan-geller though that we shouldn't exit on background task failure, unless the things people are doing with background tasks have changed substantially - data collection should not be contingent on live plotting, especially since the user can manually quit the loop if they're not seeing what they want.

Originally the code had the last_task_failed flag - the idea here was to allow the background task to fail once without being removed, following the idea "once is never, twice is always." I seem to recall seeing this with live plotting when the original draw may have had no data in it, but the second draw, when there is some data, would work? Of course that bug could be fixed, but who knows what else will come up particularly with Qt.

[giulioungaretti]

@alexcjohnson @alan-geller I am not sure I like the "once is never, twice is always." But let's not change it for now.
So no early return, log the exception and remove the task if fail twice.