[MM-15220] Add generic error field to SubmitDialogResponse (#12081)

This will allow integrations to return an generic error message that is
not tied to a specific dialog field.

model/integration_action.go

@@ -222,6 +222,7 @@ type SubmitDialogRequest struct {
 }
 
 type SubmitDialogResponse struct {
+	Error  string            `json:"error,omitempty"`
 	Errors map[string]string `json:"errors,omitempty"`
 }
 

model/integration_action_test.go

@@ -141,6 +141,7 @@ func TestSubmitDialogRequestToJson(t *testing.T) {
 func TestSubmitDialogResponseToJson(t *testing.T) {
 	t.Run("all fine", func(t *testing.T) {
 		request := SubmitDialogResponse{
+			Error: "some generic error",
 			Errors: map[string]string{
 				"text":  "some text",
 				"float": "1.2",