Handle Trampoline hops in error decryption

Rather than solely iterating over `RouteHop`s, we now also append the
shared secrets from the inner onion containing `TrampolineHop`s.

Additionally, we allow the `outer_session_priv` to be overridden to
accommodate the test vector requirements. We do so by separating out a
façade method without the override.

Author: arik-so