Previously, the `NotFound` status code was used to signal many kinds of
recoverable, forwarding errors. This included validation errors, incorrect
Content-Type errors, and more.
This commit modifies the status code used to forward in these instances to more
precisely indicate the forwarding condition. In particular:
* Parameter `FromParam` errors now forward as 422 (`UnprocessableEntity`).
* Query paramater errors now forward as 422 (`UnprocessableEntity`).
* Use of incorrect form content-type forwards as 413 (`UnsupportedMediaType`).
* `WebSocket` guard now forwards as 400 (`BadRequest`).
* `&Host`, `&Accept`, `&ContentType`, `IpAddr`, and `SocketAddr` all forward
with a 500 (`InternalServerError`).
Additionally, the `IntoOutcome` trait was overhauled to support functionality
previously offered by methods on `Outcome`. The `Outcome::forward()` method now
requires a status code to use for the forwarding outcome.
Finally, logging of `Outcome`s now includes the relevant status code.
Resolves#2626.
The new examples directory...
* Contains a `README.md` explaining each example.
* Consolidates examples into more complete chunks.
* Is just better.
Resolves#1447.
Sergio Benitez