elm_icon_external: cleanup ERR msg in error case
elm_icon_external: cleanup ERR msg in error case

Summary:
Setting the group without checking it exists causes a errormessage, this
is pretty annoying for people which want to set a standart icon. And
give a better errormessage if the standart setting failed.

Reviewers: raster, cedric

Reviewed By: cedric

Differential Revision: https://phab.enlightenment.org/D2686

Signed-off-by: Cedric BAIL <240633aa59d25638de9800ef43a88ad2e208d24d@osg.samsung.com>

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