Should methods like MatrixClient.getRooms() throw a warning/error if client was not started?

I lost quite some time with a bug that was caused by me calling `MatrixClient.createRoom` then `MatrixClient.getRooms` without having called `MatrixClient.startClient` (as a result the client was not syncing and not updating its room list, even though the call to `createRoom` did return successfully with a room Id).

I think that, unless there *are* some use cases where it makes sense to call `MatrixClient.getRooms` on a client that has not been started yet, the method should probably throw an error or a warning if the client was not started.

This would apply to every method that requires the client to be synchronizing.

Start with the MatrixClient.createRoom, MatrixClient.getRooms, and MatrixClient.startClient entry points described in the issue, then identify other methods that require synchronization. Done means calls made before the client is started consistently produce the agreed warning or error behavior.

Written by the indexing model from the issue text.