Form ajax errors are swallowed by canvas

Problem

When embedding a form in canvas (e.g. via webform block), a broken ajax submission may be swalloed. The page is displayed with an ugly error on the component, and cached like this.

The is related issue about caching exceptions: #3591845

** Reproduction **

Add webform block to canvas, open the route with "?ajax_form=1" appended

Proposed resolution

Instead of swalling the form ajax error, let it pass through, like working ajax requests.