Handle undefined QLowEnergyController error cases as Unknown error

This is important on Android as not all error conditions are exposed
via QLowEnergyController::Error enum.

Change-Id: I442ecffcadc01e1a2b60ae17dc6e63e08e5f4149
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@theqtcompany.com>
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>

src/bluetooth/qlowenergycontroller.cpp

@@ -226,6 +226,7 @@ void QLowEnergyControllerPrivate::setError(
         break;
     case QLowEnergyController::NoError:
         return;
+    default:
     case QLowEnergyController::UnknownError:
         errorString = QLowEnergyController::tr("Unknown Error");
         break;