Use enum to encode error reason in code

[JoeHowarth]

In many places we return an error reason number when it's not clear whether we should revert or not immediately and need to pass the information for the failure up to calling code to do control flow.

Example: https://github.com/wormhole-foundation/trustless-generic-relayer/blob/main/ethereum/contracts/coreRelayer/CoreRelayer.sol#L157

These internal reason numbers should be turned into enums.