auth/standard_role_manager: Add exception handling for background task
auth/standard_role_manager: Add exception handling for background task

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>
Message-Id: <20171130233851.32827-1-a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

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