Module error 

Layered error types for x402 facilitator operations.

The hierarchy is:

FacilitatorError
├── Verification(VerificationError)
├── Settlement(SettlementError)
├── Aborted { reason, message }   // hook-level veto
├── Onchain(String)               // chain RPC / tx revert
└── Internal(Box<dyn Error>)      // unexpected internal failure

Every leaf carries enough information to round-trip through the wire via AsPaymentProblem, which chain-specific crates implement for their own error variants.

Enums

ClientError

Client-side error returned when constructing / signing payments.

FacilitatorError

Top-level facilitator failure.

SettlementError

Settlement-phase failures.

VerificationError

Verification-phase failures.