[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"`
 }