Update the error code when a canonical alias that does not exist is used

[clokep]

Per the clarification in the spec (see matrix-org/matrix-spec-proposals#2432 (comment)) the situation of an alias not existing should be treated the same as an alias that points to the wrong room.

Fixes #7105

See updated SyTests: matrix-org/sytest#837

[anoadragon453]

lgtm

[richvdh]

lgtm, modulo the possible nits below

[richvdh]

have you checked that this codepath is reached when it's a remote alias? I wouldn't be surprised if we fail to turn the HTTP 404 into a SynapseError.

[clokep]

I have not checked that. Will look into it!

[clokep]

I believe this is properly handled:

synapse/synapse/handlers/directory.py

Lines 240 to 264 in 229eb81

	try:
	result = yield self.federation.make_query(
	destination=room_alias.domain,
	query_type="directory",
	args={"room_alias": room_alias.to_string()},
	retry_on_dns_fail=False,
	ignore_backoff=True,
	)
	except CodeMessageException as e:
	logging.warning("Error retrieving alias")
	if e.code == 404:
	result = None
	else:
	raise
	if result and "room_id" in result and "servers" in result:
	room_id = result["room_id"]
	servers = result["servers"]
	if not room_id:
	raise SynapseError(
	404,
	"Room alias %s not found" % (room_alias.to_string(),),
	Codes.NOT_FOUND,
	)

That code seems to handle the 404, never setting a room_id, which then raises the SynapseError.