Analyze Panic error on some paymaster tx's

[rube-de]

Issue for #16 (comment)

Sometimes paymaster tx's fail con sapphire with a panic error.
It probably is a timing, but needs to be investigated further.

failed txs:
https://explorer.oasis.io/mainnet/sapphire/tx/68493f783c486e8dd9aad1c4c589d7ceafa212d9789be2b825ad06559e363962
https://explorer.oasis.io/mainnet/sapphire/tx/cc27095255522f5a2c7a42f027784074ec567e6edcc5f633fdbf8eeb5f45cbc5
https://explorer.oasis.io/mainnet/sapphire/tx/289abd5db51acf642897d1569185274b222b1eda24b88b00fa19de0625a0ca39