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

spec/controllers/api/salmon_controller_spec.rb

@@ -46,8 +46,8 @@ RSpec.describe Api::SalmonController, type: :controller do
         post :update, params: { id: account.id }
       end
 
-      it 'returns http success' do
-        expect(response).to have_http_status(202)
+      it 'returns http client error' do
+        expect(response).to have_http_status(400)
       end
     end
   end