KPluginFactory: report errors from QPluginLoader

This makes error messages more helpful for debugging.

Example:

kf.coreaddons: "Could not load plugin from /home/jpontaoski/kde/usr/lib64/plugins/kisspages/libpage_user.so: Die Bibliothek /home/jpontaoski/kde/usr/lib64/plugins/kisspages/libpage_user.so kann nicht geladen werden: (/home/jpontaoski/kde/usr/lib64/plugins/kisspages/libpage_user.so: undefined symbol: _ZN4Page16staticMetaObjectE)

(after)

vs

kf.coreaddons: "Could not load plugin from /home/jpontaoski/kde/usr/lib64/plugins/kisspages/libpage_user.so"

(before)