Propagate room join errors to the UI

Dispatch so we can set the state in RoomViewStore. Show the error
when the room join fails (unsure if it's better to do this from
the component or the store). Remove unused joinError from roomview.

src/components/structures/RoomView.js

@@ -185,7 +185,6 @@ module.exports = React.createClass({
             roomLoading: RoomViewStore.isRoomLoading(),
             roomLoadError: RoomViewStore.getRoomLoadError(),
             joining: RoomViewStore.isJoining(),
-            joinError: RoomViewStore.getJoinError(),
         }, () => {
             this._onHaveRoom();
             this.onRoom(MatrixClientPeg.get().getRoom(this.state.roomId));