Better error messages if smaller rescaling intervals needed

[hyanwong]

In #415, re setting a smaller number of rescaling intervals to avoid errors, @nspope wrote:

We'll add a more informative error message

I'm not sure which error message to change. The issue above fails on assert np.all(np.diff(rescaled_breaks) > 0) whereas #450 fails on piecewise_scale_posterior. Nevertheless, an error message which suggests a solution in both cases could be helpful.