Make EventHandler.get_event return None when the requested event is not found

[anoadragon453]

EventHandler.get_events docstring says:

Returns:
An event, or None if there is no event matching this ID.

But this is not currently true. If the event does not exist, this method will currently raise a NotFoundError. All callers of this method seem to follow the docstring, leading to incorrect behaviour.

I found this while investigating ReportEventRestServlet, which would end up returning:

{
    "errcode": 404,
    "error": "Could not find event $abcde:example.com"
}

instead of the intended:

{
    "errcode": 404,
    "error": "Unable to report event: it does not exist or you aren't able to see it."
}

hence the slight test case modification.

---

Although there are a few different callers of EventHandler.get_event, the only other implication this fix seems to have is that aggregating threads for events may be less buggy:

synapse/synapse/handlers/relations.py

Lines 354 to 358 in 41f127e

	# If the root event is not found, something went wrong, do
	# not include a summary of the thread.
	event = await self._event_handler.get_event(user, room_id, event_id)
	if event is None:
	continue

synapse/synapse/handlers/relations.py

Lines 381 to 391 in 41f127e

	# The *last* event returned is the one that is cared about.
	event = await self._event_handler.get_event(
	user, room_id, potential_events[-1].event_id
	)
	# It is unexpected that the event will not exist.
	if event is None:
	logger.warning(
	"Unable to fetch latest event in a thread with event ID: %s",
	potential_events[-1].event_id,
	)
	continue

[DMRobertson]

Happy if CI is. (I'm assuming we already deal with this returning None at every call site, in order to appease mypy.)