Use Custom Errors To Save Gas

[nmlinaric]

The smart contracts currently use require and revert string messages for error handling. However,
using a revert with a custom Error, instead of a string error message, considerably reduces the smart
contract size and optimizes gas costs on deployment. Contracts:
Bridge -> _onlyAllowed
ERC20Safe -> _safeCall

Implementation details

Use a revert with a custom Error to handle errors.

Testing details

unit tests

Acceptance Criteria

existing unit tests passing with new implementation