Fix fs.unlink incorrect exception when file not exists

src/ios/RNFetchBlob/RNFetchBlob.m

     NSError * error = nil;
     NSString * tmpPath = nil;
     [[NSFileManager defaultManager] removeItemAtPath:path error:&error];
-    if(error == nil)
+    if(error == nil || [[NSFileManager defaultManager] fileExistsAtPath:path] == NO)
         callback(@[[NSNull null]]);
     else
         callback(@[[NSString stringWithFormat:@"failed to unlink file or path at %@", path]]);