smp: always return exceptions as failed futures in smp::submit_to()
smp: always return exceptions as failed futures in smp::submit_to()

Make it simpler for callers to handle exceptions.

xtao/seastar,tempbottle/seastar,flashbuckets/seastar,stamhe/seastar,xtao/seastar,printedheart/seastar,dwdm/seastar,stamhe/seastar,kjniemi/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,acbellini/seastar,flashbuckets/seastar,xtao/seastar,leejir/seastar,scylladb/scylla-seastar,bowlofstew/seastar,sjperkins/seastar,bowlofstew/seastar,scylladb/scylla-seastar,avikivity/seastar,anzihenry/seastar,cloudius-systems/seastar,leejir/seastar,tempbottle/seastar,norcimo5/seastar,dreamsxin/seastar,scylladb/seastar,dwdm/seastar,syuu1228/seastar,printedheart/seastar,dreamsxin/seastar,scylladb/scylla-seastar,anzihenry/seastar,flashbuckets/seastar,kjniemi/seastar,shyamalschandra/seastar,wildinto/seastar,bowlofstew/seastar,kjniemi/seastar,shyamalschandra/seastar,chunshengster/seastar,chunshengster/seastar,mixja/seastar,jonathanleang/seastar,jonathanleang/seastar,acbellini/seastar,leejir/seastar,linearregression/seastar,syuu1228/seastar,koolhazz/seastar,cloudius-systems/seastar,dreamsxin/seastar,linearregression/seastar,joerg84/seastar,wildinto/seastar,tempbottle/seastar,hongliangzhao/seastar,norcimo5/seastar,jonathanleang/seastar,stamhe/seastar,joerg84/seastar,wildinto/seastar,joerg84/seastar,printedheart/seastar,hongliangzhao/seastar,koolhazz/seastar,chunshengster/seastar,cloudius-systems/seastar,hongliangzhao/seastar,shyamalschandra/seastar,dwdm/seastar,anzihenry/seastar,mixja/seastar,mixja/seastar,norcimo5/seastar,linearregression/seastar,sjperkins/seastar,avikivity/seastar,acbellini/seastar,koolhazz/seastar,scylladb/seastar,sjperkins/seastar