Fix 22947 - Use wenforce in sysErrorString

[MoonlightSentinel]

This ensures that the method throws a WindowsException initialised
with the proper error code. Also avoids unrelated exceptions that
could suppress the error which caused the call to sysErrorString.

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Related: #8420

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22947	minor	sysErrorString throws Exception instead of WindowsException

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8422"