Don't log "success" when failing over to original image

Closes #311 (closed)

We use fail-over logic in the image_resizer where if certain conditions aren't met or scaling fails before writing the first byte, we serve the original image instead. Let's call them "soft-failures".

However, so far we always logged "ImageResizer: Success" in either case, which is confusing because those soft-failures will be "counted" as successes, which they aren't.

This MR changes the logging logic slightly by logging soft-failures explicitly as such.