core/gate: Ensure with_gate leaves gate on exception
core/gate: Ensure with_gate leaves gate on exception

This path makes with_gate exception safe.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>
Message-Id: <20180202230553.29333-1-a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

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