Throwing while adding to the timeline will result in all the messages in the backfill or load-from-storage missing from the timeline

E.g. https://github.com/vector-im/element-meta/issues/1390 was an image in with no `content.url` which threw in ImageTile, blocking 10 or so messages from ever being displayed, without a clear error.

We should have a mechanism where we render the tile with `errorToDOM` or something similar.

Also we should have a safe function to read fields/paths of an event, and act predictably when that field is missing/wrong type/ ...