outbound: Add error type to TCP and TLS routes (#387)

This PR adds explicit error types to TLS and TCP routes and backends. They serve two distinct purposes: 

- a route level error is to be used when we want to fail all traffic that goes through a particular route (i.e. blocked by default traffic)
- a backend level error is to be used when we want to fail traffic for just a single backend on a route (i.e. in situations when we have more than one backends but some of them have caused `InvalidKind` reasons).

This behavior is governed by the Gateway API GEP described here: https://gateway-api.sigs.k8s.io/geps/gep-1364/

Signed-off-by: Zahari Dichev <zaharidichev@gmail.com>