Better error message when a remote resource uses invalid Content-Type

[anoadragon453]

I noticed while working on #8421 that Synapse will raise a wrapped RuntimeError and 500 when a remote server responds to a request with a Content-Type header that Synapse was not expecting.

This is a tiny PR to change it to a 502 (Bad Gateway) instead, which I think makes more sense. I also spruced up the error message text a bit.

[erikjohnston]

I think we probably want to fix this by not returning a 500 in response to a RequestSendFailed?

[anoadragon453]

In this case the upstream server is at fault. What should we return instead to the user?

[erikjohnston]

I think we generally use 502 for such things. E.g.:

synapse/synapse/rest/media/v1/media_repository.py

Lines 387 to 394 in 6abb1ad

	except RequestSendFailed as e:
	logger.warning(
	"Request failed fetching remote media %s/%s: %r",
	server_name,
	media_id,
	e,
	)
	raise SynapseError(502, "Failed to fetch remote media")

The general approach is that:

1. we don't know whether SynapseError is appropriate for what the caller is trying to do; and
2. we should always be handling RequestSendError when making http requests, as otherwise we'll be generating 500s and ERROR logs for situations which aren't bugs in the server.

[anoadragon453]

@erikjohnston Thanks for the explanation! Fixing this will need to happen in other places then. For now I've changed this PR to just update the error message to be slightly clearer.