Pass original validation error if boom

[hueniverse]

Previously, hapi would always override any validation error and return a 400. This changes it to keep the error object if it is a boom error with its own status code and pass that instead. Same error decorations apply.

It is considered a bug because it should have honored the returned error with its own status code already. It is an enhancement because it allows new use cases with returning custom errors in validation rules.

Is it marked as breaking because it might cause custom code not using joi rules but one that does use boom errors to return different error codes. However, it will still return an error and therefore this is published as a minor release, not a major.