Velo: Add warning message if rawbank is empty

Improve error reporting, for cases like Moore!1703 (merged)

When a wrong rawbank type is asked for (for instance VP insted of VPRetina), default_raw_banks() happily returns an empty rawbank, which is then skipped by the Velo algorithm. With this MR, the Velo algorithm will report the empty rawbank so it can be caught in tests and by the user.

@sponce @cmarinbe