Catch SystemError in portable code

[Ericson2314]

Motivation

This will ensure this catching works on Windows too, not just Unix.

Context

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[edolstra]

Does this work? Above it's throwing a SysError with an errno, but where does the errno get converted to an std::errc that can be caught here?

[xokdvium]

We now store a std::error_code (system_category) and it's comparable to a std::errc even on windows since fd0bcd9. In unix-like system_category is the same as generic_category, which is just the errno. On windows the comparison goes through the first overload of equivalent virtual function https://en.cppreference.com/w/cpp/error/error_category/equivalent.html, which implements a mapping from windows errors to POSIX ones (std::error_condition is implicitly constructible from std::errc: https://en.cppreference.com/w/cpp/error/error_condition/error_condition.html that uses the generic_category: https://en.cppreference.com/w/cpp/error/errc/make_error_condition.html)

[xokdvium]

Looks good to me.