Introduction of revert error codes

[akolotov]

Currently the bridge contracts do not use the revert messages to clarify the reason of reverts. That's why sometimes it is hard to guess the exact reason why the bridge operations were reverted. At the same time, the revert reasons started supporting by the block explorers like etherscan and blockscout that's why it makes sense to provide the messages to the users there, explaining why the revert happened.

Since introduction of the string messages as the revert reasons could require intensive storage usage, it is worth to consider to use error codes that could consume less gas.