messaging_service: Add missed throw
messaging_service: Add missed throw

It is missed somehow.
Message-Id: <3df465d3e0a0b3fabca27ec081ec60962be6b0a9@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla,kjniemi/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,kjniemi/scylla,raphaelsc/scylla,kjniemi/scylla,raphaelsc/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla