Fix HTTP responses for salmon and ActivityPub inbox processing (#5200)

* Return sensible HTTP status for ActivityPub inbox processing

* Return sensible HTTP status for salmon slap processing

* Return additional information to debug signature verification failures

app/controllers/activitypub/inboxes_controller.rb

@@ -9,9 +9,9 @@ class ActivityPub::InboxesController < Api::BaseController
     if signed_request_account
       upgrade_account
       process_payload
-      head 201
-    else
       head 202
+    else
+      [signature_verification_failure_reason, 401]
     end
   end