Refs #4468
A typo'd or invented --scope matched no label at all, and because
nothing carries a label in a scope that does not exist, every open
item came back reported as missing it -- the failure direction reads
as a finding instead of an error.
Validate --scope the same way list --label already validates
--label: reuse labels::repo_labels (the paginated label fetch) and
error with the bad scope plus the available ones, using the same
with_suggestions near-miss helper list --label's message uses.
ded0379f97