Log icon class when it fails

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Check also for icon.props.names absence.

Affects also volumes toolbar icons. We need to
figure out where to put this _find_icon_name
method, so we do not repeat all over.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>