When an error returned by they REST API is shown to the user in the admin or account console not all details are shown. For example, when adding an execution to a built-in flow the following message is displayed:

However, looking at the error response from the API much more detail is present that is relevant to the user:

{
  "error": "It is illegal to add execution to a built in flow",
  "error_description": "For more on this error consult the server log at the debug level."
}

These details should be shown in the alert as well, so that both the error and error_description fields are presented to the user.