Improve output on PRIV_TASK/TOP in init/fini.
Improve output on PRIV_TASK/TOP in init/fini.

Output a proper error message instead of a more cryptic assert
if the vmod writer does the wrong thing.

Ref: #1811

feld/Varnish-Cache,feld/Varnish-Cache,gquintard/Varnish-Cache,feld/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,feld/Varnish-Cache,gquintard/Varnish-Cache,feld/Varnish-Cache