st_widget_get_theme_node: g_error if called on an unparented widget

It's wrong to do anything that requires looking up a widget's style
before you add the widget to the stage, since its final style may
depend on properties inherited from its parents.
st_widget_get_theme_node() used to emit a warning in this case, but
many would-be contributors apparently didn't notice. Help them out.

https://bugzilla.gnome.org/show_bug.cgi?id=610279