std.mmfile: Improve error handling

Use wenforce instead of (inappropriately) errno.
Switch from if blocks to scope blocks.

Needs ~~https://github.com/D-Programming-Language/phobos/pull/2505~~
Ah, you meant the signedness, not the size of the type. I guess it should technically be an uint, but since we're not doing any operations where the sign matters, it won't make a difference even despite the hardware limitations.