ui: fixed Linux issues

* no outline style for push buttons
* show error when not using Sourcetrail.sh script in console instead of message box, which won't load

src/app/main.cpp

@@ -187,6 +187,13 @@ QCoreApplication* createApplication(int &argc, char *argv[], bool noGUI = false)
 
 int main(int argc, char *argv[])
 {
+#ifdef __linux__
+	if (std::getenv("SOURCETRAIL_VIA_SCRIPT") == nullptr)
+	{
+		std::cout << "ERROR: Please run Sourcetrail via the Sourcetrail.sh script!" << std::endl;
+	}
+#endif
+
 	QApplication::setApplicationName("Sourcetrail");
 	QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling, true);