[vitormattos]

Refs LibreSign/libresign#8117

Summary

• Document the error-handling convention in a short Developer Manual page: exception types, semantic codes, explicit HTTP statuses, and the legacy fallback.
• Record existing signing frontend contracts that must survive incremental migrations.
• Explain when tests should assert exception types, codes, messages, and final HTTP/OCS responses.
• Link the page from the architecture map and Developer Manual navigation.

Validation

• Reviewed the 71-line reStructuredText page for structure and line length.
• Verified the commit's cryptographic signature and matching DCO sign-off.
• Sphinx was not installed in the local execution environment; the repository's Documentation workflow will validate the build with warnings treated as errors.

Scope

Documentation only. No application changes, blanket removal of message assertions, or migration of legacy status codes.