[Docs] Document all error codes in the payment contract

[fati-Onchain]

The payment contract defines numeric error codes (in contracts/payment/src/lib.rs) that are not documented anywhere. Add a table listing each error code, its symbolic name, and what condition triggers it.

Acceptance criteria

• Error table added to contracts/payment/README.md (or a new contracts/payment/ERRORS.md)
• Every error variant in the contract's error enum is covered

[Chigybillionz]

@Chigybillionz has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hi, I’ve reviewed this issue and I’m confident I can fix it. I’d like to take it on.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Chigybillionz to this issue.

[drips-wave]

Congratulations, @Chigybillionz! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @Chigybillionz: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox!

@Chigybillionz's PR #456 was approved and merged by @fati-Onchain.

🏆 @Chigybillionz: You earned 40 FoxPoints for this contribution! Your current tier: Builder (1,699 total points). Track your full progress on GrantFox.

👏 Great work, @Chigybillionz! Keep contributing to Facil-Pay.

[drips-wave]

This issue has been marked as completed by a Drips Wave moderator for @Chigybillionz as part of the Stellar Wave Program's 7th Wave 🥳

Moderator's reason: [Auto-assessed] The PR was successfully merged by maintainer @fati-Onchain on 2026-07-25T15:44:08Z, as confirmed by the bot comment. The contribution directly addresses the issue by creating a comprehensive ERRORS.md file documenting over 100 error variants across multiple categories.

😎 @Chigybillionz: You earned 150 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here.

🧑‍💻 Repo maintainers: How'd the contributor do? Leave a review to share your experience working with them.

ℹ️ Note: This issue was marked complete via moderation. Points were issued even though the GitHub issue remains open.