Was going to add all kinds of logic to handle the situation where the
source page count was less than a single signature, but I realize that
was both too complicated and presumed that the program knew what was the
"right-thing-to-do" (tm). 

Instead, the program logs an error and stops; the user should have (and
want) more control about how the layout is handled (e.g. pad the front
or back of the source with blank, or possibly watermarked, pages).