scope: refuse activation of scopes if no PIDs to add are left

scope: refuse activation of scopes if no PIDs to add are left

If all processes we are supposed to add are gone by the time we are
ready to do so, let's fail.

THis is heavily based on Cunlong Li's work, who thankfully tracked this
down.

Replaces: #20577
(cherry picked from commit 8d3e4ac7cd37200d1431411a4b98925a24b7d9b3)
(cherry picked from commit 7ecb1b82d9b55a081d81b2802695fd21293ce029)