Change OnionFailureCode returned when failing trampoline forwarding

[f321x]

This changes the OnionFailureCode returned from maybe_forward_trampoline() on PaymentFailure to TRAMPOLINE_FEE_INSUFFICIENT so the client will retry the payment with higher trampoline fee instead of giving up.

[ecdsa]

Note: This reverts eda9097. That commit is related to a discussion in lightning/bolts#829

The FIXME indicates that there are some cases where it would make sense to return unknown_next_peer.
Specifically:

• we should return TRAMPOLINE_FEE_INSUFFICIENT if we could not find a route because of too low fees
• we should return UNKNOWN_NEXT_PEER if we cannot find the next trampoline in the graph (per Trampoline Routing (2021 edition) (Feature 56/57) lightning/bolts#829 (comment) )