[G-1] Use custom errors rather than revert()/require() strings to save gas

[juniormp]

Comments

Custom errors are available from solidity version 0.8.4. Custom errors save ~50 gas each time they’re hit by avoiding having to allocate and store the revert string. Not defining the strings also save deployment gas.

Instance

https://github.com/juniormp/audit-2023-04-rubicon/blob/511636d889742296a54392875a35e4c0c4727bb7/contracts/BathHouseV2.sol#L27
https://github.com/juniormp/audit-2023-04-rubicon/blob/511636d889742296a54392875a35e4c0c4727bb7/contracts/BathHouseV2.sol#L33
https://github.com/juniormp/audit-2023-04-rubicon/blob/511636d889742296a54392875a35e4c0c4727bb7/contracts/BathHouseV2.sol#L68
https://github.com/juniormp/audit-2023-04-rubicon/blob/511636d889742296a54392875a35e4c0c4727bb7/contracts/BathHouseV2.sol#L144