Add a convenience function that is like gtk_builder_get_object()
but stashes away a GError if a lookup fails. To make the error
message informative, the function takes a line/column pair.

Doing things this way is necessary because the custom_tag_end,
custom_finished, and parser_finished vfuncs don't take a
GError parameter, despite being called from a place where
we can report a GError back.