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.

SDWebImage/SDWebImageManager.m

@@ -112,7 +112,7 @@
                                         progress:(SDWebImageDownloaderProgressBlock)progressBlock
                                        completed:(SDWebImageCompletionWithFinishedBlock)completedBlock {
     // Invoking this method without a completedBlock is pointless
-    NSParameterAssert(completedBlock);
+    NSAssert(completedBlock != nil, @"If you mean to prefetch the image, use -[SDWebImagePrefetcher prefetchURLs] instead");
 
     // Very common mistake is to send the URL using NSString object instead of NSURL. For some strange reason, XCode won't
     // throw any warning for this type mismatch. Here we failsafe this error by allowing URLs to be passed as NSString.