routing: return nicer error message when routes fail due to the fee limit

[Roasbeef]

Today when making a payment or requesting a route, a payment can fail if hits the specified fee limit. The default value is 100% (should definitely be lowered), so a routine payment of 1 satoshi can at times fail since many nodes (on testnet at least) enforce a 1 satoshi base fee. In order to make this error more actionable for users and also GUIs, we should return a concrete error if path finding fails due to the fee limit. Atm, we just let that error go, and attempt to move unto the next route.

Related to #1662.

Steps To Completion

• In findPath, keep track of unexplored routes due to fee limit errors.

• If no route is found, then if all of our past errors were due to violating the fee limit, then return a new concrete error.

• Amend the relevant RPC servers (and possibly sub-servers) to expose this new error to the caller.

[orbitalturtle]

Taking this

[joostjager]

@Roasbeef I don't think it is as easy as the steps to completion suggest.

• Even if not all of our past errors were due to violating the fee limit, it could be that increasing the fee limit fixes the problem. There could for example be a channel temporarily unavailable error among them.
• It could be that a route is unexplored because of the fee limit, but that increasing the fee limit would still yield no route, because there is for example no connection at all further down the route.

I think we need to think carefully about our options to provide better feedback.

[joostjager]

Also there may be multiple errors causing routing to fail. For example some routes failing because of the fee limit, others because of the cltv limit and a third set because of an outgoing channel restriction. Returning a single error wouldn't cover that situation.

[saubyk]

Can this issue be closed in-lieu of Attributable errors proposal (lightning/bolts#1044)?

[saubyk]

Closing this issue, as it's better to build it based on feedback received from wallet and app developers