replication: fix gh-853 "incomprehensible error message"

When handling join, an error from box_on_cluster_join()
was ignored and not sent to the client, so the client
believed that the JOIN was handled successfully and proceeded
with SUBSCRIBE.

Before sending OK to join, execute box_on_cluster_join(),
if it fails, the client sees an error and stops.

Make ER_REPLICA_MAX a logged error.

No test case since the test requires a fail-stop test of the replica,
which is clumsy in our framework.