prevent futures ignored by parallel_for_each from generating warnings
prevent futures ignored by parallel_for_each from generating warnings

parallel_for_each() reports only first error and ignores all others.
There is not way for an application to get to ignored errors and handle
them, so ignore then properly to avoid warnings.

Message-Id: <a649b95b75aab87589d15f91d6cb570a74434468@scylladb.com>

cloudius-systems/seastar,cloudius-systems/seastar,dreamsxin/seastar,kjniemi/seastar,dreamsxin/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,cloudius-systems/seastar,syuu1228/seastar,kjniemi/seastar,scylladb/seastar,kjniemi/seastar,syuu1228/seastar,avikivity/seastar,dreamsxin/seastar,avikivity/seastar,scylladb/seastar