Override MKNetworkOperation so 4xx response body available in the error handler

SomeNetworkOperation.m

// Register your NetworkOperation class after creating your network engine
// someEngine = [[SomeEngine alloc] initWithHostName:SOME_API customHeaderFields:nil];
// [someEngine registerOperationSubclass:[SomeNetworkOperation class]];


// ------ SomeNetworkOperation.h ------
#import "MKNetworkOperation.h"
@interface SomeNetworkOperation : MKNetworkOperation
@end

// ------ SomeNetworkOperation.m ------
#import "SomeNetworkOperation.h"

@implementation SomeNetworkOperation

-(void) operationFailedWithError:(NSError*) error
{
    NSMutableDictionary *userInfo = [NSMutableDictionary dictionaryWithDictionary:error.userInfo];
    [userInfo setObject:[self responseString] forKey:@"responseString"];
    [super operationFailedWithError:[NSError errorWithDomain:NSURLErrorDomain code:error.code userInfo:userInfo]];
}

@end