Return Result values from functions

[JeffBelgum]

Most statistics functions can fail if certain invariants aren't met. These invariants must be checked against at runtime, therefore functions should return Results rather than values.

A single result type should be defined for the library with a single error type that implements the Error trait. This will be a very breaking change to the api!

[garro95]

Also, the documentation should say when a function can panic! and when it returns an Error

[JeffBelgum]

https://github.com/withoutboats/failure looks like a promising library for handling errors and result types.