return NotFoundError if room not found

Per the Client-Server API[0] we should return
`M_NOT_FOUND` if the room isn't found instead
of generic SynapseError.

This ensures that /directory/list API returns
404 for room not found instead of 400.

[0]: https://matrix.org/docs/spec/client_server/unstable.html#get-matrix-client-r0-directory-list-room-roomid



Signed-off-by: Serban Constantin <serban.constantin@gmail.com>