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~~
Why `null` and not `INVALID_HANDLE_VALUE`? What happens when you call `CreateFileMappingW` with `hFile` == `null`? Is it documented?