parsePositiveInt reported invalidity through a bool that both call sites
discarded, and returned (false, nil) whenever Atoi succeeded on a negative
number. GetLogs therefore accepted ?cursor=-5 and served it as cursor 0
('latest') instead of the documented 400. Validity now travels through the
error result, which no caller can ignore.
Five sites used == against sentinels (redis.Nil, ingest.ErrForbidden,
errs.ErrDatabaseUninitialized). The neighbouring not-found checks already
went through errors.Is helpers, so a wrapped error would silently downgrade
a 403 to a 400 and a 500 to a 400.
ryan