When trivial provider is loaded, setting 'wsrep_cluster_address' should return error

I think trivial provider should act like black hole swallow everything without feedback.

On one hand it should.
On the other, failure to load the provider may go unnoticed (e.g. API mismatch) and this is the way to notify the user that wsrep functionality is in fact missing.
Also, loading a proper provider will zero up wsrep_cluster_address and wsrep_provider_options set while the dummy provider was loaded. So returning error when user attempts to set any of these should indicate to him that these actions will have no result and he will have to repeat them after loading a proper provider.

Just to clarify - here we distinguish between setting provider-specific variables and regular database operations. Ideally, wsrep_% variables should not be visible at all when provider is not loaded - compare that to InnoDB variables which are not visible when InnoDB is disabled - but that's a far bigger task.