Add robust error handling to USDC payout flow

[AbdulSnk]

Overview

The current USDC payout flow on Stellar lacks proper error handling. Any failure during a payout transaction — network issues, insufficient funds, invalid addresses — needs to be caught, logged, and handled gracefully to protect contributors from lost or stuck rewards.

What needs to be done

• Audit the existing payout logic in the codebase
• Identify all points where a transaction can fail
• Implement error handling for the following scenarios:
  • Invalid or inactive Stellar wallet address
  • Insufficient funds in the escrow account
  • Network timeout or Stellar network unavailability
  • Transaction rejected by the Stellar network
• Log all failed transactions with enough detail to debug and retry
• Implement a retry mechanism for transient failures (e.g. network timeouts)

Acceptance Criteria

• All identified failure points have proper error handling
• Failed transactions are logged with error type, contributor, and amount
• Retry logic is implemented for transient failures
• Permanent failures (e.g. invalid address) are flagged without retrying
• Unit tests cover all error scenarios listed above

Notes

No payout should fail silently. Every error must be logged and surfaced. This is critical since Stellar transactions are irreversible — getting this right protects contributors.