Don't leave the cluster during joining flow when double join is attempted

[TheMMaciek]

Currently when a node is attempting a double join - the error is thrown that a double join was attempted. Every error in the joining flow is handled with an automatic leave from the cluster. So in case a user executes the joining flow for the second time the node leaves the cluster, which in case someone executes the joining endpoint by mistake can be annoying.

So the double join error during joining flow should be handled differently than other errors.
We need a distinguishable case class representing this error instead of a generic Throwable and then we need to handle this special case with logging only.