GEOS  subroutines that are able to return errors,  return one  of
          the following error numbers in the X register.
          1    Not enough blocks on the disk
          2    Illegal track or sector.
          3    Disk full, insufficient space.
          4    Directory full
          5    File not found.
          6    Attempt to deallocate an unallocated block.
          7    Illegal VLIR chain number.
          8    VLIR file error; illegal track or sector specified.
          9    Too many VLIR chains, out of records.
          10   File is not a VLIR file.
          11   End of file, file too long.
          13   Device not present.
          31   Write protect is on.
          32   Read error, no sync character.
          35   Disk drive FDC errors:
                    2    Header block not found.
                    7    Verify error after write.
                    9    Header block checksum error.
                    10   Data block too long.
                    11   ID mismatch error.
          37   Write verify error
          38   Disk is write protected.
          38   Disk drive FDC errors?
                    4    Data block not found.
                    5    Data block checksum error.
          39   Write error.  See VerWriteBlock.