storage_proxy: simplify error handling by using this/handle_exception
storage_proxy: simplify error handling by using this/handle_exception

It is cleaner to use this/handle_exception instead of then_wrapped if
normal and error flow do not share any state.

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