Handle as an error if WPImageSource download succeeds but image is nil.

I thought it seemed weird that an `AFImageRequestOperation` could succeed but the `UIImage` be nil. Then I recalled an earlier issue where a server was returning images with an `image/jpg` mime type where `image/jpeg` is expected. Can't say for sure if that's what's happening here. 

Here I'm treating the nil image as an error and adding some logging to help nail down the actual cause in the future. 

Fixes #1156, and probably #1154 as well.