Make fs::err_t enum a subcategory of std::error_condition; use the st…

[ajrhacker]

…andard codes in some places

[cuavas]

I think making the errors an error_condition category is a sensible change.

However, I don’t think using std::errc error conditions for errors relating to the emulated filesystem is a great idea. It makes it impossible to distinguish between errors accessing the underlying storage holding the disk image and errors from accessing the filesystem withing the disk image. If anything, I would have considered using std::errc::not_supported in place of ERR_UNSUPPORTED but kept ERR_NOT_FOUND, ERR_NOT_EMPTY and ERR_NO_SPACE equivalents in the filesystem error category.

[cuavas]

Is this extra printf intentional? It looks like the kind of thing that would get spammy fast.

[ajrhacker]

It's a leftover from an unrelated debugging effort. I'll remove it.