`network.cpp` has several error paths which either:
- report "Unhandled host socket error=n" and return `SUCCESS`, or
- switch on a few possible errors, log them, and translate them to
Errno; the same switch statement is copied and pasted in multiple
places in the code
Convert these paths to use a helper function `GetAndLogLastError`, which
is roughly the equivalent of one of the switch statements, but:
- handling more cases (both ones that were already in `Errno`, and a few
more I added), and
- using OS functions to convert the error to a string when logging, so
it'll describe the error even if it's not one of the ones in the
switch statement.
- To handle this, refactor the logic in `GetLastErrorMsg` to expose a
new function `NativeErrorToString` which takes the error number
explicitly as an argument. And improve the Windows version a bit.
Also, add a test which exercises two random error paths.
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