rs/SDWebImage — pull request #854 by Gabro (2014-08-12 09:23:44+00:00)
Title: More explicit error message in case of nil completedBlock

NSParameterAssert doesn't provide enough information for understanding the API misuse.
Pointing the API user to SDWebImagePrefetcher makes more sense, as a nil completedBlock most likely signifies the intention of prefetching the image.