auth::password_authenticator: Ensure exceptions are processed in continuation
auth::password_authenticator: Ensure exceptions are processed in continuation

Fixes #1718 (even more)
Message-Id: <fcb8c04379e0d388ec845e0b210e6fc018b50216@scylladb.com>

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