Disc errors returned by the IDE driver.
Where they can be mapped onto a similar error from the ST506 driver,
the error code is shown.

&02  IVC  command aborted by controller
&07  WFL  write fault
&08  NRY  drive not ready
&09  NSC  track 0 not found
&13  DFE  uncorrected data error
&16  TOV  sector id field not found
&17  NIA  bad block mark detected
&18  NDA  no data address mark
&20       no DRQ when expected
&21       drive busy when commanded
&22       drive busy on command completion
&23       controller did not respond within timeout
&24       unknown code in error reg
