If an error occurs deserializing, regrouping, serializing, or writing data to disk, and the shuffle isn't in "backpressure mode", that data will simply be lost and the shuffle will still succeed

When task isn't awaited, the asyncio task is leaked. Any error that occurred in it is also lost (besides being logged).

@graingert and I tried to fix this, but we couldn't get something working:

• The dumb way (keep leaking tasks, wrap all of receive in a try/except, set self._exception, raise self._exception in add_partition, get_output_partition, inputs_done) fails tests because of leaking tasks
• The "proper" asyncio way causes a CancelledError to pop out in some unexpected place and seems to shut down the whole worker?

xref #6201