Allow request guards to customize error responses

[pjenvey]

Request guards lack the ability to customize error Responses on Failures other than setting a Status code. Error catchers can further customize error responses afterwards but they lack the context of the original error that occurred, which could be important for the response.

E.g. a request guard fails to validate some input and wants to render a JSON response including an accompanying error message.

It seems the only way to handle this is to defer to the Handler (having it take a Result<> from the guard). It would be much more convenient if a Responder could some how be passed along to Failure cases.

[SergioBenitez]

Yes, this is absolutely a sore point. I've been working on a revamped error management design that will alleviate the issue. When it's ready, I'll post an issue describing it.

[SergioBenitez]

Let's track this in #749.