[network] Error handling reform

`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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