make sure all distributions throw exceptions at undefined boundaries #603
Description
Go through all the distributions and make sure that they throw exceptions at boundaries. Also change all the doc so it's clear about positive vs. non-negative. For example, scale parameter sigma must be positive for normal distribution and prior count parameters alpha and beta must be positive for beta distribution.
Right now, I think some of the distributions try to be clever, like defining normal(mu,0) to be a discrete distribution with 100% of its mass at mu. Don't!