Use Error::asStdString if available

This fixes wrongly encoded translated error descriptions on Windows
when called from the main thread (e.g. for displaying an error in the UI
for the user). For secondary threads one still gets error descriptions
in native encoding instead of UTF-8 on Windows, but that should only
affect debug logs.

(cherry picked from commit d8810e25e55cfa04398bf37434f087a78f4795ff)

• GnuPG-bug-id: T5960