message: Display common errors with QMessageBox
  Beforehand, these errors were only printed with qDebug(). Display
  them with QMessageBox in addition in order better communicate
  issues.
diff: @@ -130,7 +130,9 @@ void MongoChemWidget::finishSearch(const QVariantMap& results)
  auto resultList = results.value("results").toList();
  int matches = resultList.size();
  if (matches == 0) {
  - qDebug() << "No results found!";
  + QString message = "No results found!";
  + qDebug() << message;
  + QMessageBox::critical(this, "MongoChem", message);
  return;
  }
  @@ -154,7 +156,9 @@ void MongoChemWidget::downloadSelectedMolecule()
  {
  int row = selectedRow();
  if (row < 0) {
  - qDebug() << "No molecule selected!";
  + QString message = "No molecule selected!";
  + qDebug() << message;
  + QMessageBox::critical(this, "MongoChem", message);
  return;
  }
  