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~~
(Note: this isn't new code, it just appears in the diff due to indentation changes.) But yes, `CreateFileMapping` takes its 64-bit size parameter as two 32-bit parameters (high part and low part).