If someone knows what `20, READ ERROR` means, then that one should recognize that `NO SECTOR HEADER` means the same thing.  Yes, one might need "to think twice" because the new messages might be unexpected; but, I don't think that they will be confusing.

`READ ERROR (NO SECTOR HEADER)` would be more familiar; but, I want the messages to be short.  Hopefully, we won't see them often enough to justify verbose text that bloats programs.