clients aren't told if they are trying to speak in a server_acl'd room

This issue has been migrated from [#3618](https://github.com/matrix-org/synapse/issues/3618).

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we should put checks on the CS APIs to give nice errors if a client tries to talk in a room where their server has been server_acl'd, rather than shouting into the void. there's the general problem that if other servers in the federation aren't accepting your stuff you don't know you're shouting into the void, but we can at least handle this better.