Consolidate Win32-specific error formatting

GetErrorReason()'s Win32 implementation does the same thing as
Win32ErrorString(int err) from syserror.cpp, so call the latter.

Also remove now-unnecessary headers from sock.cpp and less verbose
handling of #ifdefs.

src/util/fs.cpp

@@ -81,12 +81,7 @@ bool FileLock::TryLock()
 #else
 
 static std::string GetErrorReason() {
-    wchar_t* err;
+    return Win32ErrorString(GetLastError());
-    FormatMessageW(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,
-        nullptr, GetLastError(), MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), reinterpret_cast<WCHAR*>(&err), 0, nullptr);
-    std::wstring err_str(err);
-    LocalFree(err);
-    return std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>>().to_bytes(err_str);
 }
 
 FileLock::FileLock(const fs::path& file)

src/util/sock.cpp

@@ -15,11 +15,6 @@
 #include <stdexcept>
 #include <string>
 
-#ifdef WIN32
-#include <codecvt>
-#include <locale>
-#endif
-
 #ifdef USE_POLL
 #include <poll.h>
 #endif
@@ -416,15 +411,12 @@ void Sock::Close()
     m_socket = INVALID_SOCKET;
 }
 
-#ifdef WIN32
 std::string NetworkErrorString(int err)
 {
+#if defined(WIN32)
     return Win32ErrorString(err);
-}
 #else
-std::string NetworkErrorString(int err)
-{
     // On BSD sockets implementations, NetworkErrorString is the same as SysErrorString.
     return SysErrorString(err);
-}
 #endif
+}