Fix webfinger retries (#4275)

* Do not raise unretryable exceptions in ResolveRemoteAccountService

* Removed fatal exceptions from ResolveRemoteAccountService

Exceptions that cannot be retried should not be raised. New exception
class for those that can be retried (Mastodon::UnexpectedResponseError)

spec/workers/pubsubhubbub/delivery_worker_spec.rb

@@ -26,7 +26,7 @@ describe Pubsubhubbub::DeliveryWorker do
       subscription = Fabricate(:subscription)
 
       stub_request_to_respond_with(subscription, 500)
-      expect { subject.perform(subscription.id, payload) }.to raise_error(/Delivery failed/)
+      expect { subject.perform(subscription.id, payload) }.to raise_error Mastodon::UnexpectedResponseError
     end
 
     it 'updates subscriptions when delivery succeeds' do