widget: Add error message when the parent parameter was set wrongly.
widget: Add error message when the parent parameter was set wrongly.

This helps application developers debug their apps easier.

tasn/elementary,FlorentRevest/Elementary,FlorentRevest/Elementary,tasn/elementary,FlorentRevest/Elementary,rvandegrift/elementary,tasn/elementary,rvandegrift/elementary,tasn/elementary,FlorentRevest/Elementary,rvandegrift/elementary,rvandegrift/elementary,tasn/elementary