`lightning-liquidity`: Introduce better error types

[tnull]

In lightning-liqudity we historically at some point went with reusing LDK's APIError type, which was a weird choice even back then.

However, we since added a bunch of additional error cases, and at the very least since #4008 (where we had to use APIError for inline persistence errors), APIError makes no sense anymore.

We should introduce a dedicated error type with clearly documented variants for lightning-liquidity and use it throughout the LSPS1/2/5 APIs.