Return error to RP instead of user for prompt=none

app/controllers/api/openid_connect/authorizations_controller.rb

@@ -219,8 +219,8 @@ module Api
 
       def auth_user_unless_prompt_none!
         if params[:prompt] == "none" && !user_signed_in?
-          render json: {error:       "login_required",
-                        description: "User must be first logged in when `prompt` is `none`"}
+          handle_params_error("login_required",
+                              "User must already be logged in when 'prompt' is 'none'")
         else
           authenticate_user!
         end

spec/controllers/api/openid_connect/authorizations_controller_spec.rb

@@ -141,8 +141,7 @@ describe Api::OpenidConnect::AuthorizationsController, type: :controller do
         it "should return an interaction required error" do
           post :new, client_id: client.client_id, redirect_uri: "http://localhost:3000/",
                response_type: "id_token", scope: "openid", state: 1234, display: "page", prompt: "none"
-          json_body = JSON.parse(response.body)
-          expect(json_body["error"]).to match("login_required")
+          expect(response.location).to match("error=login_required")
         end
       end