Issue41

The original version of code is correct (6.0-30) for knnimpute, for most users who have error messages such as 
Error in nn2(old[, cols, drop = FALSE], new[, cols, drop = FALSE], k = k) : 
  NA/NaN/Inf in foreign function call (arg 1)
or
 Error in nn2(old[, cols, drop = FALSE], new[, cols, drop = FALSE], k = k) : 
  no points in data!
The possible cause is that there are factor variables which have been treated as numeric ones, when preProcess() do the scale, it will generate NAs in object$data. I have added one error message to detect those issues in predict.preProcess()