improve error reporting for getStats functions

[MatthewWhiting]

The way the getStats functions report / deal with errors is not terribly useful. It would be better to make use of a return value so that the calling function knows if something went wrong.

Here's the example that I had: I was calling findMedianStats with a mask, but had the wrong size provided - instead of the size of the array, it was the number of good points in the mask. findMedianStats then looks at only the first 'size' points in the mask array, but in this case they were invalid, and so no good points were found.

Might also want to improve the documentation so that the nature of the "size" parameter is explicitly identified.