Errors are not displayed in the user settings form

While testing the form along with backend!2325 (merged), only the usual error notifications were displayed but the form's error handling was never activated because this.fieldErrors is not updated to the errors returned by the backend.

When trying to patch this, I found that the template expects all field error arrays to always be set, but that isn't the case. For example, the backend might only return {"password": […]}, so fieldErrors.display_name would not exist.