Allow spam checkers to override the error message

[gabrc52]

Description:

For instance in 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.

[clokep]

It looks like #13044 almost allows this, but it only lets you return additional fields, the message is hard-coded:

synapse/synapse/handlers/room_member.py

Line 945 in 62a1a9b

"Not allowed to join this room",