When datadir missing, show messagebox instead of printing error to stderr

src/qt/bitcoin.cpp

@@ -184,7 +184,10 @@ int main(int argc, char *argv[])
     // ... then bitcoin.conf:
     if (!ReadConfigFile(mapArgs, mapMultiArgs))
     {
-        fprintf(stderr, "Error: Specified directory does not exist\n");
+        // This message can not be translated, as translation is not initialized yet
+        // (which not yet possible because lang=XX can be overridden in bitcoin.conf in the data directory)
+        QMessageBox::critical(0, "Bitcoin",
+                              QString("Error: Specified data directory \"%1\" does not exist.").arg(QString::fromStdString(mapArgs["-datadir"])));
         return 1;
     }