Add reporting for validation errors coming from widgets

[westonruter]

Just as we are now detect the specific shortcode that is the source for an AMP validation error in #971 (and as we'll do for blocks in Gutenberg), we also need to do the same for widgets. When a widget renders something that not valid AMP, we need to indicate:

1. The widget that did something invalid.
2. The theme/plugin source for where the widget came from.