Renaming ErrorHandler to ErrorHandlerMiddleware

[boesing]

This re-opens zendframework/zend-stratigility#191 and handles #2

Summary

As already mentioned in zendframework/zend-stratigility#190 (migrated to #3), there is a naming conflict in the ErrorHandler.

I had some feedback of trainees which asked me, why the ErrorHandler ain't a RequestHandler, as its name says.
That made me think about the naming and I realized, its just because of the missing Middleware postfix.

However, I'd like to rename the ErrorHandler to ErrorHandlerMiddleware to avoid future confusions about where the difference between RequestHandlerInterface and MiddlewareInterface is.

[Xerkus]

I disagree with the change. Name does not say that it is a request handler but that it is an error handler, it handles errors.

While Handler might seem somewhat ambiguous, none of the other middlewares follow proposed naming convention and namespace already clearly indicates that it is a middleware.

[boesing]

@Xerkus As I've already tried to explain in zendframework/zend-stratigility#190, its not about what we as the creators of this package think is "enough" but what my workshop attendees do.
I had multiple developers from juniors to seniors which asked me about the ErrorHandler and why the handler does not end the middleware pipeline as it states to be a Handler (and handlers break chains as they do not forward the request to the next handler).
If you use a use statement to simplify the namespace, what stays in your pipeline configuration is the ErrorHandler which (for me and many others) states to be a handler.

I understand why you think so but I still think this is a useful change to make things more clear.

Just a sidenote: the NotFoundHandler is definitely a handler which does not delegate the request to the next handler and still is in the Middleware namespace. So it has to be a middleware? 🤷‍♂
Fix for that is in #8