multishard_mutation_query: do_query: prevent exceptions using coroutine::as_future
multishard_mutation_query: do_query: prevent exceptions using coroutine::as_future

Optimize error handling by preventing
exception try/catch using coroutine::as_future.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla