Bug#892060: Modifying archived bugs via blocks should fail before starting to block, rather than in the middle

[Don Armstrong <don@debian.org>]

Package: debbugs
Severity: normal

While putting blocking into #885135, #885310 was archived, so all
subsequent bugs were not properly blocked.

This should either fail early, and block no bugs, or skip the archived
bugs, and block the rest of the bugs.

-- 
Don Armstrong                      https://www.donarmstrong.com

[T]he question of whether Machines Can Think, [...] is about as
relevant as the question of whether Submarines Can Swim.
 -- Edsger W. Dijkstra "The threats to computing science"