Grub Error Code 15: "Specified" File not Found
sounds like your upgrade went astray somewhere. Either it changed the file name in Grub and not the actual file name or Vice versa. In which case reinstalling from scratch may have been your best (or only) option.
Here is the GRUB manual, numbered codes are addressed in section 14.3 http://www.gnu.org/software/grub/manual/grub.html
I'll look into writing a GRUB Tutorial but the big issue I see with doing any Linux tutorial is that things change from one distro to the next so you never really get good coverage of an issue.