gms/gossiper: Check for shadow round completion before throwing
gms/gossiper: Check for shadow round completion before throwing

For values of `shadow_round_ms` lower than 1 second, this was assuming
failure without checking.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

duarten/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla