Text Widget: Warnings thrown when using invalid HTML in legacy mode

[jeherve]

Steps to reproduce the issue

1. Start with a site running WP 4.7.10.
2. Add a new Text Widget to your site with the following contents:

<img src="http://example.com/img.jpg" border="0" title="Example" /></a>

3. Update to WP 4.9.5
4. Notice the widget still there under Appearance > Widgets.
5. Load your site's front end.
6. In your logs, you should see the following warning:

[Suppressed] Warning: DOMDocument::loadHTML(): Unexpected end tag : a in Entity, line: 1 in /home/wpcom/public_html/wp-includes/widgets/class-wp-widget-text.php on line 129

This seems to happen because of the invalid HTML in the widget.

In #41622 and r41251, we suppressed those warnings. Unfortunately invalid HTML can still fill in logs. I think it would be nice if we could get rid of those warnings, with the attached patch.

Let me know what you think about this!

[jeherve]

Text Widget: suppress all warnings thrown when using invalid HTML in legacy mode

[westonruter]

👍

[pento]

In 44578:

Widgets: Don't log invalid HTML in the Text Widget.

[41215] supresses these irrelevant error messages from the front end, this removes them from the error log, too.

Props jeherve.
Fixes #43815.