Fix 22946 - Mark WindowsException ctor as nothrow

[MoonlightSentinel]

Catch possible exceptions arising from e.g. UTF decoding and ensure that the message buffer doesn't contain partial output from a failed step.

The logic is seperated into a dedicated method because it's required for another bugfix.

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Extracted from #8420.

CC @CyberShadow.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22946	normal	WindowsException ctor is not nothrow

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#8421"

[CyberShadow]

Great, thank you!

[MoonlightSentinel]

(Rebased to hopefully fix buildkite ...)