Handle NaN results from Presto server

with comments addressed (throw an exception when we see a non-NaN double in the result while we were expecting a string)
Nit: I think this is pretty self explanatory. One also need not use a function for NaN to be a valid value.

I'm of the philosophy that in-line comments are most often a sign that more names should be introduced into the code itself. (though that applies to other cases, it just makes me weary of in-line comments in general)