Move reading of multipart response into try body

[anoadragon453]

Otherwise the exception will be raised outside of the error handling code. Fixes #19061.

Missed in #17365.

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[MadLittleMods]

Should this be inside the async with self.remote_download_linearizer.queue(ip_address):?

Maybe. We do it in get_file which is probably what federation_get_file was based off of:

synapse/synapse/http/matrixfederationclient.py

Lines 1578 to 1582 in da6c0ca

	async with self.remote_download_linearizer.queue(ip_address):
	# add a byte of headroom to max size as function errs at >=
	d = read_body_with_max_size(response, output_stream, expected_size + 1)
	d.addTimeout(self.default_timeout_seconds, self.reactor)
	length = await make_deferred_yieldable(d)

And although this isn't explained anywhere, I guess the point of self.remote_download_linearizer is to ensure we only download 6 pieces of remote media at a time for some reason (maybe resource exhaustion, just a bunch of assumptions). So it makes sense that the download part is under the lock.

[MadLittleMods]

Do we have tests for this kind of timeout? (just wondering)

It looks like this was previously functional and the only problem was the unexpected logs/error handling so an high-level integration/end-to-end test wouldn't have really prevented this.

It looks like we already have other tests in tests/http/test_matrixfederationclient.py that ensure we raise a RequestSendFailed in certain scenarios. We could add one of those 🤷