Currently, the ThumbnailRenderJob pulls the status of requests to Thumbor out of the request object, and does it's own logic to decide whether a request was successful. This is due to Thumbor occasionally returning codes such as 400 in response to requests that are (arguably, perhaps) successful -- such as a request to generate a thumbnail that's larger than the source image.

T200524 suggests enhancing MWHttpRequest such that an indication can be given that other statuses are valid. If that merges, then the job should be updated to use that.