DisplayComponent: explicitly log setDisplayMode() success

It can be deduced from the other log messages, but that's always such a
drag.

src/display/DisplayComponent.cpp

@@ -134,7 +134,12 @@ bool DisplayComponent::switchToBestVideoMode(float frameRate)
       << m_displayManager->displays[currentDisplay]->videoModes[bestmode]->getPrettyName()
       << "on display" << currentDisplay;
 
-      return m_displayManager->setDisplayMode(currentDisplay, bestmode);
+      if (!m_displayManager->setDisplayMode(currentDisplay, bestmode))
+      {
+        QLOG_INFO() << "Mode switching failed.";
+        return false;
+      }
+      return true;
     }
     QLOG_INFO() << "No better video mode than the currently active one found.";
   }