make-secnet-sites: Crash if complain() is called too late

Every call to complain() is supposed to occurs before the code in the
main program which checks `complaints'.  But maybe there is an
erroneous late call, or one may be introduced.  In this case it is
important to crash, because otherwise bad data might end up being
written into our output.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>