Forward to original URL on image error
Currently if an error occurs during image processing (due to image size, format, invalid/unreachable origin) the error returned to the browser is:
{"error":{"name":"invalid_image"}}
My suggestion is to just forward the request to the source image URL with a HTTP 307. This will make the user experience better, more so on recent posts where images are usually still available.
This change should not interfere with future fixes that will hopefully reduce the number of those errors. It's just the default behaviour of the system in case an image cannot be served.