lwt: do not return unavailable exception from the 'learn' stage
lwt: do not return unavailable exception from the 'learn' stage

Unavailable exception means that operation was not started and it can be
retried safely. If lwt fails in the learn stage though it most
certainly means that its effect will be observable already. The patch
returns timeout exception instead which means uncertainty.

Fixes #7258

Message-Id: <0c7bb7e1cba143fdb86e00259de25979ba5fbb80@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla