Ensure that incoming to-device messages are not dropped (#17127)

... when workers are unreachable, etc.

Fixes https://github.com/element-hq/synapse/issues/17117.

The general principle is just to make sure that we propagate any
exceptions to the JsonResource, so that we return an error code to the
sending server. That means that the sending server no longer considers
the message safely sent, so it will retry later.

In the issue, Erik mentions that an alternative solution would be to
persist the to-device messages into a table so that they can be retried.
This might be an improvement for performance, but even if we did that,
we still need this mechanism, since we might be unable to reach the
database. So, if we want to do that, it can be a later follow-up.

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Co-authored-by: Erik Johnston <erik@matrix.org>

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