propagate errors during async refresh

[viqtor]

resolves #636

[Sunjeet]

Thanks for this contribution. Changing the return type of the triggerAsyncRefresh method from void to CompletableFuture<Void> would cause binary backwards incompatibility. Is there a reason to not simply use the synchronous flavor triggerRefresh/triggerRefreshTo in your application?

[viqtor]

Is there a reason to not simply use the synchronous flavor triggerRefresh/triggerRefreshTo in your application?

We have been following the reference implementation. We have since switched to the sync alternative since dicsovering this issue but it may lead to false assumptions when you pass in a refreshExectutor to your consumer that isn't used.

I just noticed that the same method is also used during the getInitialLoad() which wont trip the catch block.

Would an alternative solution be to deprecate these methods and add new ones with the new return type?