Get Win32 error messages with FormatMessageW instead of FormatMessageA.

Also include the error number in the string even if there is a message, because otherwise looking them up is hard.

Also remove the trailing newline from the message.

c++/src/kj/debug.c++

@@ -34,6 +34,7 @@
 #define NOIME 1
 #include <windows.h>
 #include "windows-sanity.h"
+#include "encoding.h"
 #endif
 
 namespace kj {
@@ -357,26 +358,30 @@ void Debug::Fault::init(
     const char* file, int line, Win32Error osErrorNumber,
     const char* condition, const char* macroArgs, ArrayPtr<String> argValues) {
   LPVOID ptr;
-  // TODO(soon): Use FormatMessageW() instead.
   // TODO(soon): Why doesn't this work for winsock errors?
-  DWORD result = FormatMessageA(FORMAT_MESSAGE_ALLOCATE_BUFFER |
+  DWORD result = FormatMessageW(FORMAT_MESSAGE_ALLOCATE_BUFFER |
                                 FORMAT_MESSAGE_FROM_SYSTEM |
                                 FORMAT_MESSAGE_IGNORE_INSERTS,
                                 NULL, osErrorNumber.number,
                                 MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
-                                (LPTSTR) &ptr, 0, NULL);
+                                (LPWSTR) &ptr, 0, NULL);
 
+  String message;
   if (result > 0) {
     KJ_DEFER(LocalFree(ptr));
-    exception = new Exception(typeOfWin32Error(osErrorNumber.number), file, line,
-        makeDescriptionImpl(SYSCALL, condition, 0, reinterpret_cast<char*>(ptr),
-                            macroArgs, argValues));
+    const wchar_t* desc = reinterpret_cast<wchar_t*>(ptr);
+    size_t len = wcslen(desc);
+    if (len > 0 && desc[len-1] == '\n') --len;
+    if (len > 0 && desc[len-1] == '\r') --len;
+    message = kj::str('#', osErrorNumber.number, ' ',
+        decodeWideString(arrayPtr(desc, len)));
   } else {
-    auto message = kj::str("win32 error code: ", osErrorNumber.number);
-    exception = new Exception(typeOfWin32Error(osErrorNumber.number), file, line,
-        makeDescriptionImpl(SYSCALL, condition, 0, message.cStr(),
-                            macroArgs, argValues));
+    message = kj::str("win32 error code: ", osErrorNumber.number);
   }
+
+  exception = new Exception(typeOfWin32Error(osErrorNumber.number), file, line,
+      makeDescriptionImpl(SYSCALL, condition, 0, message.cStr(),
+                          macroArgs, argValues));
 }
 #endif