New ErrorHandler middleware.

This middleware catches exceptions, logs them, and then re-executes the request to generate an error page. The request can be re-executed down the original pipeline with a modified request path, or down an alternate pipeline.  If anything goes wrong generating the response then the original exception will be re-thrown.

The re-execution will not be attempted if the response has already started.

@loudej @davidfowl @Eilon 
`Clear()` I would  expect to be part of the buffering feature, but we haven't designed it yet.

Maybe I should just take this line out for now.