Add exception handling to directory servlet, so we don't 500. Mark directory API as volatile in the api docs.

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

change the world: make the default matrix API URL prefix /_matrix rather than /matrix to make it easier for existing websites to mount a HS in their namespace without collisions.

perl -pi -e 's#/matrix#/_matrix#g' ./cmdclient/console.py ./docs/client-server/howto.rst ./docs/client-server/specification.rst ./docs/client-server/swagger_matrix/directory ./docs/client-server/swagger_matrix/events ./docs/client-server/swagger_matrix/login ./docs/client-server/swagger_matrix/presence ./docs/client-server/swagger_matrix/profile ./docs/client-server/swagger_matrix/registration ./docs/client-server/swagger_matrix/rooms ./docs/server-server/specification.rst ./graph/graph.py ./jsfiddles/create_room_send_msg/demo.js ./jsfiddles/event_stream/demo.js ./jsfiddles/example_app/demo.js ./jsfiddles/register_login/demo.js ./jsfiddles/room_memberships/demo.js ./synapse/api/urls.py ./tests/federation/test_federation.py ./tests/handlers/test_presence.py ./tests/handlers/test_typing.py ./tests/rest/test_events.py ./tests/rest/test_presence.py ./tests/rest/test_profile.py ./tests/rest/test_rooms.py ./webclient/components/fileUpload/file-upload-service.js ./webclient/components/matrix/matrix-service.js

api docs: Finished adding all C-S APIs. Added initialSync, publicRooms, membership changes (generic and RPCy) and directory paths.