Previously, the result of when_all was discarded. when_all stores
exceptions in the returned futures rather than throwing, so the outer
catch(in_use&) could never trigger. Now we capture the when_all result
and inspect each future individually to properly detect in_use from
either stream.
Fixes https://scylladb.atlassian.net/browse/SCYLLADB-1216Closesscylladb/scylladb#29219