Add more specific error messages to HTTP signature verification (#21617)

* Return specific error on failure to parse Date header

* Add error message when preferredUsername is not set

* Change error report to be JSON and include more details

* Change error report to differentiate unknown account and failed refresh

* Add tests

app/services/activitypub/fetch_remote_actor_service.rb

@@ -28,6 +28,7 @@ class ActivityPub::FetchRemoteActorService < BaseService
     raise Error, "Unsupported JSON-LD context for document #{uri}" unless supported_context?
     raise Error, "Unexpected object type for actor #{uri} (expected any of: #{SUPPORTED_TYPES})" unless expected_type?
     raise Error, "Actor #{uri} has moved to #{@json['movedTo']}" if break_on_redirect && @json['movedTo'].present?
+    raise Error, "Actor #{uri} has no 'preferredUsername', which is a requirement for Mastodon compatibility" unless @json['preferredUsername'].present?
 
     @uri      = @json['id']
     @username = @json['preferredUsername']