check for changed sections that are not included

There are many cases where a section may have
changed due to soure-level change but the inclusion
logic has not selected it for output.  Some of these
cases are real no-go situations like changing data
structures.  Some are just situations that
create-diff-object isn't smart enough to figure out
(yet).

Either way, it should be considered fatal when a
changed section hasn't been selected for output.

Signed-off-by: Seth Jennings <sjenning@redhat.com>