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~~
I've seen it's is not new, thanks to (finally!) having split-view on github.

It's old code and nobody can physically map more then 2^63 bytes of data on any modern CPUs ( x86_64 have up to ~ 2^48 of virtual memory range, currently).