ignore all but one failed futures in map_reduce
ignore all but one failed futures in map_reduce

map_reduce runs a lot of mappers in parallel, but only error from the
first one to fail is propagated to a caller. Ignore all other mapper's
results explicitly if this happens, otherwise if another one fails the
warning about ignored exception future will be printed.

Message-Id: <22f91aa708f11ecad4442fbfa33af365023163ae@scylladb.com>

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