Currently we print an ERROR on all exceptions in
`raft_topology_cmd_handler`. This log level is too high, in some cases
exceptions are expected -- like during shutdown. And it causes dtest
failures.
Turn exceptions from aborts into WARN level.
Also improve logging by printing the command that failed.
Fixesscylladb/scylladb#19754Closesscylladb/scylladb#19935