Allow spam checkers to override the error message

This issue has been migrated from [#16259](https://github.com/matrix-org/synapse/issues/16259).

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**Description:**

For instance in [user_may_join_room](https://matrix-org.github.io/synapse/latest/modules/spam_checker_callbacks.html#user_may_join_room)


It is currently possible to override the status code / error code but not the error message. It would be useful for plugin devs to specify to users trying to join a room why they were denied for the room and what they can do in order to be granted access.

Clients like Element can then then display that error message.

Start with the user_may_join_room spam-checker callback documentation linked in the issue, then trace how its status and error code reach the client. Done means spam-checker plugins can provide a user-facing denial message that clients such as Element can display, with the callback behavior and compatibility covered.

Written by the indexing model from the issue text.