Renaming ErrorHandler to ErrorHandlerMiddleware

[weierophinney]

As already mentioned in #190, 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.

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Originally posted by @boesing at zendframework/zend-stratigility#191

[weierophinney]

/cc @weierophinney @webimpress

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Originally posted by @boesing at zendframework/zend-stratigility#191 (comment)

[weierophinney]

@boesing I think there might be some references to this class in the documentation so we need update it. Other than that LGTM 👍

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Originally posted by @michalbundyra at zendframework/zend-stratigility#191 (comment)

[boesing]

Can be closed in favor of #7

[boesing]

Closed as this wont be applied.