AppletQuickItem: Fix weird format of error messages

The previous message was containing a user-visible string and was missing context, for example:
kf.plasma.quick: QJsonValue(string, "Printers")
kf.plasma.quick: QJsonValue(array, QJsonArray([".../org.kde.plasma.printmanager/contents/ui/main.qml:56:34: Type FullRepresentation unavailable","... : module \"QtQuick.Controls\" version 1.3 is not installed"]))

Now, one gets:
kf.plasma.quick: error when loading applet "org.kde.plasma.printmanager" QList("... Type FullRepresentation unavailable", "...: module \"QtQuick.Controls\" version 1.3 is not installed")

Not having the QJsonValue/QJsonArray prefix reduced mental overhead,
because this is only the internal representation and not useful
information in addition to the error.