Don't return `failureDetails` for `invoice could not be paid` error

[kilrau]

Example:

{
  "status": "invoice.failedToPay",
  "failureReason": "invoice could not be paid",
  "failureDetails": {
    "actual": 3925137,
    "expected": 3925134
  }
}

This is misleading as one could think the swap failed due to the slight overpayment.