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)

app/lib/exceptions.rb

@@ -5,4 +5,14 @@ module Mastodon
   class NotPermittedError < Error; end
   class ValidationError < Error; end
   class RaceConditionError < Error; end
+
+  class UnexpectedResponseError < Error
+    def initialize(response = nil)
+      @response = response
+    end
+
+    def to_s
+      "#{@response.uri} returned code #{@response.code}"
+    end
+  end
 end