Change the optional method into required, force the custom loader author to provide the error check

SDWebImage/SDImageLoader.h

@@ -86,11 +86,9 @@ FOUNDATION_EXPORT UIImage * _Nullable SDImageLoaderDecodeProgressiveImageData(NS
                                               completed:(nullable SDImageLoaderCompletedBlock)completedBlock;
 
 
-@optional
 /**
  Whether the error from image loader should be marked indded un-recoverable or not.
  If this return YES, failed URL which does not using `SDWebImageRetryFailed` will be blocked into black list. Else not.
- If does not implements this method, assume always return NO.
 
  @param url The URL represent the image. Note this may not be a HTTP URL
  @param error The URL's loading error, from previous `requestImageWithURL:options:context:progress:completed:` completedBlock's error.

SDWebImage/SDImageLoadersManager.m

@@ -104,11 +104,7 @@
     NSArray<id<SDImageLoader>> *loaders = self.loaders;
     for (id<SDImageLoader> loader in loaders.reverseObjectEnumerator) {
         if ([loader canRequestImageForURL:url]) {
-            if ([loader respondsToSelector:@selector(shouldBlockFailedURLWithURL:error:)]) {
-                return [loader shouldBlockFailedURLWithURL:url error:error];
-            } else {
-                return NO;
-            }
+            return [loader shouldBlockFailedURLWithURL:url error:error];
         }
     }
     return NO;

SDWebImage/SDWebImageManager.m

@@ -374,11 +374,7 @@ static id<SDImageLoader> _defaultImageLoader;
     if ([self.delegate respondsToSelector:@selector(imageManager:shouldBlockFailedURL:withError:)]) {
         shouldBlockFailedURL = [self.delegate imageManager:self shouldBlockFailedURL:url withError:error];
     } else {
-        if ([self.imageLoader respondsToSelector:@selector(shouldBlockFailedURLWithURL:error:)]) {
-            shouldBlockFailedURL = [self.imageLoader shouldBlockFailedURLWithURL:url error:error];
-        } else {
-            shouldBlockFailedURL = NO;
-        }
+        shouldBlockFailedURL = [self.imageLoader shouldBlockFailedURLWithURL:url error:error];
     }
     
     return shouldBlockFailedURL;

Tests/Tests/SDWebImageTestLoader.m

@@ -50,4 +50,8 @@
     return task;
 }
 
+- (BOOL)shouldBlockFailedURLWithURL:(NSURL *)url error:(NSError *)error {
+    return NO;
+}
+
 @end