Faster joins: stuck "partial state room" if persisting the `/send_join` response fails

[richvdh]

If the response to /send_join turns out to be invalid (so processing it fails), we end up with a room stuck in the "partial state" state.

part of #12646

[richvdh]

synapse/synapse/handlers/federation.py

Lines 545 to 549 in 7c6b220

	if ret.partial_state:
	# TODO(faster_joins): roll this back if we don't manage to start the
	# background resync (eg process_remote_join fails)
	# https://github.com/matrix-org/synapse/issues/12998
	await self.store.store_partial_state_room(room_id, ret.servers_in_room)

[richvdh]

I think this is relatively low-hanging fruit which someone could pick up without too much introduction