Improve d64 handling

[jepler]

I was trying to use fluxengine d64 files with vice x64, but I found that when there were error sectors the disk image written out simply omitted them.

This caused vice to refuse to recognize the d64 file, because only a few specific file lengths are accepted. This made vice reject the floppy disk image with "unknown gcr version".

Instead, when a sector is not present that should have been, write out zeros and append an error bitmap.

I have used several pieces of software archived from floppy disks (using the adafruit prototype floppy featherwing) in x64 with this patch: Jumpman, Alf, and "Pro Financial Organizer" (hey, when you borrow whatever floppies a friend will lend you, you get weird stuff).

[jepler]

This may need more work, I've found that it doesn't seem to append the error map when I expect it to.

[davidgiven]

OK, let me know when you want me to have another look.