feat(addListener): throw an error if next, error or complete functions are missing
feat(addListener): throw an error if next, error or complete functions are missing

This class of error is a compiler error in TypeScript, but is a runtime
error in JS. Currently we get an error like "this._n is not a function"
which is quite cryptic to an end user.

staltz/xstream,andersonleite/xstream,andersonleite/xstream,andersonleite/xstream,staltz/xstream,staltz/xstream,staltz/xstream