Fix: Check if desktop file from first argument really exists

and issue a proper error message if not.

When using linuxdeployqt for example in OBS and packages can not
be installed, it can happen that the desktop file is not existing.
This error message makes debugging easier.

tools/linuxdeployqt/main.cpp

@@ -88,6 +88,12 @@ int main(int argc, char **argv)
          * to do when using linuxdeployqt. */
         if (firstArgument.endsWith(".desktop")){
             qDebug() << "Desktop file as first argument:" << firstArgument;
+
+            /* Check if the desktop file really exists */
+            if (! QFile::exists(firstArgument)) {
+                LogError() << "Desktop file in first argument does not exist!";
+                return 1;
+            }
             QSettings * settings = 0;
             settings = new QSettings(firstArgument, QSettings::IniFormat);
             desktopExecEntry = settings->value("Desktop Entry/Exec", "r").toString().split(' ').first().split('/').last().trimmed();