Fix /upload 500'ing when presented a very large image (#10029)

* Fix /upload 500'ing when presented a very large image

Catch DecompressionBombError and re-raise as ThumbnailErrors

* Set PIL's MAX_IMAGE_PIXELS to match homeserver.yaml

to get it to bomb out quicker, to load less into memory
in the case of super large images

* Add changelog entry for 10029