On special float value error, provide path in object

When serializing objects, I sometimes get the special float value error because of a `NaN` or `Infinity`. It's expected, but the error message, while accurate, doesn't specify where in my object is the offending value.

Having the full path (e.g. `l1obj.l2obj.l3arr[27].offending`) in the error message would be a great help when dealing with complex objects. If not possible, then at least the last key / index would be better than nothing.

What do you think?