Update to support rocket =0.5.0-rc.4

[kije]

First off, thanks for this great crate! :)

Rocket 0.5.0-rc.4 was just released with several breaking changes.

Relevant for rocket-validation seems to be:

• Outcome::Failure was renamed to Outcome::Error.
• The status codes used when built-in guards forward were changed. (relevant for tests)

[somehowchris]

@kije @huangjj27 Thanks, sry, had a some personal things, so shortcomings in my gh-life, but v0.2.0 should have all the changes.