When counter mutation is about to be sent, a leader is elected, but
if the leader fails after election, we get `rpc::closed_error`. The
exception propagates high up, causing all connections to be dropped.
This patch intercepts `rpc::closed_error` in `storage_proxy::mutate_counters`
and translates it to `mutation_write_failure_exception`.
References #2859
d87d01b501