Contracts use require statements instead of custom errors

[idrisssystem]

Is your feature request related to a problem? Please describe.
Some of our contracts still use require, instead of custom error messages. As pointed out by @DevABDee in #7, this can lead to higher gas fees than necessary. More info can be found here.

Describe the solution you'd like
Clean our contracts by introducing custom error messages wherever it is beneficial. Please provide relevant screenshots on performed tests and compare before <--> after gas values.

Additional context
For questions, please ask here or join the discussion on Discord.

[TradMod]

Really Glad to see this =D
Let's try to make every freakin contract gas optimized until the ETH Surge 💯

[TradMod]

Payments.sol Fixed. PR #16 :)