Elaborate error message for amp-script with attribute data-ampdevmode

In #27076, we made it legal to use `data-ampdevmode` on an `` component even if the attribute was not included on the root html node. An example:

```html

...

...

```

This approach works! However, it throws a validation error in the Chrome extension, even though it doesn't throw one in the Console:

`The attribute 'data-ampdevmode' in tag 'amp-script' is set to the invalid value ''.`