apidocs: Tweak join response format. Explicitly state empty JSON objects where they are required by the spec. Mark unimplemented room GET APIs clearly.

Conflicts:
	synapse/http/server.py
        synapse/http/content_repository.py

Edited /presence REST servlet to raise SynapseErrors to return a standard error response, rather than a string.

Better indicate in the page feedback section that a user can't join a room (because he has been banned for ex). May be worth to be put in the middle of screen