[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.

src/core/network/network.h

@@ -7,6 +7,7 @@
 #include <array>
 #include <utility>
 
+#include "common/common_funcs.h"
 #include "common/common_types.h"
 
 namespace Network {
@@ -21,6 +22,11 @@ enum class Errno {
     MFILE,
     NOTCONN,
     AGAIN,
+    CONNREFUSED,
+    HOSTUNREACH,
+    NETDOWN,
+    NETUNREACH,
+    OTHER,
 };
 
 /// Address families