Add ERR_INVALIDMODEPARAM for responding to invalid mode params.

Add ERR_INVALIDMODEPARAM for responding to invalid mode params.

Currently on invalid modes we do a combination of different things:

1. Send a custom mode-specific numeric (which often collides with
   other modes).

2. Send a server notice.

3. Do absolutely nothing.

This new numeric is a generic way of handling invalid parameters
when setting a mode that avoids all of the mistakes of the previous
behaviour.