Unify error handling behavior for subscriptions.

[dpasek-senacor]

The error handling behavior between persistent and non-persistent subscriptions is inconsistent and partly erroneous:

• Persistent subscriptions do not handle the "requires leader" case.
• Non persistent subscriptions call onError() twice in the case of NotLeaderException

This pull requests unifies the behavior and avoids duplicate calls to the error listener.

[YoEight]

Nice catch by the way!

[dpasek-senacor]

@YoEight

Please stop making my life easier

😂

Maybe just as an explanation: we are currently preparing the migration of our EventStoreDB integration from the old (unofficial) ESJC (see: https://github.com/msemys/esjc no longer actively developed as TCP protocol is deprecated) to your new gRPC Java client. Our integration has quite an extensive test coverage and therefore it is quite easy for us to find issues in the EventStoreDB client. As we want to proceed with the migration it is also easier for me to provide the patches myself instead of writing issue reports or feature requests and hoping things will be done by other people. 😉