Return legacy image format if webp does not exist

Problem/Motivation

If for any reason generation of the webp file fails the response is just "Error generating image."
This results in pages with broken images.

Instead the failure should be logged, and an image returned in legacy format, so at least the page doesn't look broken.

Steps to reproduce

Look at an image on a page after the webp generation has failed.

Proposed resolution

If the webp file is not available then log a failed webp generation, and then defer the request to the parent download controller deliver method.