validate header should raise an exception instead of returning none

@yakobchuk I think validate_header should raise an error instead of a warning if you try to validate a header for an unknown instrument. I think it's okay to fail hard here because the function simply can't o anything if it doesn't know what to check for. In this case it should fail spectacularly instead of returning None and exiting quietly... In the latter case it's pretty hard to track down the bug.

o something like: RaiseNotImplementedError("The instrument [NAME] is not implemented here yet") should do the trick.