On windows, show message box with help, as there is no stderr (fixes #702)

(partial)

src/init.cpp

@@ -241,7 +241,12 @@ bool AppInit2(int argc, char* argv[])
 
         // Remove tabs
         strUsage.erase(std::remove(strUsage.begin(), strUsage.end(), '\t'), strUsage.end());
+#if defined(QT_GUI) && defined(WIN32)
+        // On windows, show a message box, as there is no stderr
+        wxMessageBox(strUsage, "Usage");
+#else
         fprintf(stderr, "%s", strUsage.c_str());
+#endif
         return false;
     }