src: guard against nullptr returned by uv_err_name

In Electron, uv_err_name returns nullptr on unknown system errors, which V8 tries to create a string from and then crashes.

This PR introduces a guard against err_name being set to null by setting it to UnknownSystemError if nullptr is returned by uv_err_name and thus preventing the V8 crash.

Checklist

• make -j4 test passes
• commit message follows commit guidelines