`on_create_room` Module API callback encourages using `SynapseError` to deny creating a room

This issue has been migrated from [#11123](https://github.com/matrix-org/synapse/issues/11123).

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We had a discussion recently more or less along the lines of 'modules shouldn't know about `SynapseError`s (except for use with their own web resources)', but I just noticed that the `on_create_room` callback encourages/requires doing it that way.

Should we add an alternative? (e.g.: `return False, "failure message"`?)

https://github.com/matrix-org/synapse/blob/5e0e6835416776e4d938f53b3c9a005970f88127/docs/modules/third_party_rules_callbacks.md?plain=1#L68-L69

Read docs/modules/third_party_rules_callbacks.md at the on_create_room callback description and review how it currently describes denying room creation with SynapseError. Define an alternative rejection contract that keeps module callbacks independent of SynapseError, then update the callback API and its documentation; the issue does not name tests or implementation files.

Written by the indexing model from the issue text.