FileUtil: Use errno for printing errors in IsDirectory

stat() returns an error code in errno on both POSIX compliant
platforms and Windows.

This means we should always use errno instead of GetLastErrorMsg
which uses GetLastError() (Win32) on Windows.

Source/Core/Common/FileUtil.cpp

@@ -78,8 +78,7 @@ bool IsDirectory(const std::string& filename)
 
   if (result < 0)
   {
-    WARN_LOG(COMMON, "IsDirectory: stat failed on %s: %s", filename.c_str(),
-             GetLastErrorMsg().c_str());
+    WARN_LOG(COMMON, "IsDirectory: stat failed on %s: %s", filename.c_str(), strerror(errno));
     return false;
   }