Fix error handling post-migration to immutable messages

[weierophinney]

Following the change to use immutable messages, error handlers largely stopped working. This was due to the fact that they were not receiving the current message instances -- and thus could not pull up-to-date data from them.

This patch does the following:

• FinalHandler now no longer accepts the request/response instances to the constructor, but rather expects them to __invoke as the second and third methods -- just like normal error middleware, but without the final callable.
• Next now passes the current request/response instances to error handlers.
• Next now passes the current request/response instances to the final handler.
• Next now also allows passing both the request and response when specifying an error.

One upshot: unless you are performing processing AFTER calling $next(), you should return its return value to ensure the chain completes properly. This, in the end, is a more functional paradigm, as the chain and application no longer rely on side effects, but explicit return values.