A request path with an invalid percent escape, such as "/bucket/%zz", failed url.PathUnescape in the DecodeURL middleware and the raw error was answered with 500 InternalError. S3 rejects such a request with 400 InvalidURI. Return InvalidURI from DecodeURL, so the s3 and admin routers answer with it as well, and let the website handler pass the error through instead of mapping it on its own. Fixes #2070