Enhance the callback.mdx with more error handling examples

The callback.mdx file provides a basic overview of setting up a callback endpoint, but it could be greatly improved by adding more robust error handling examples.

Tasks:

1. Expand the existing code example to include more comprehensive error handling:
   • Add try/catch blocks to handle potential exceptions
   • Include examples of handling network errors, timeouts, and invalid data
   • Show how to gracefully degrade functionality if the callback fails
2. Add examples of handling different types of errors that might occur during proof verification
3. Include a section on logging and monitoring callback errors for debugging purposes
4. Provide examples of how to communicate errors back to the user in a user-friendly way
5. Add a troubleshooting section that lists common callback issues and their solutions
6. Include best practices for securing the callback endpoint against potential attacks

These enhancements will help developers create more robust and reliable implementations of Reclaim Protocol.

Contributors: Please note that by submitting a pull request, you are agreeing to accept and sign the Contributor License Agreement (CLA) found at https://github.com/reclaimprotocol/.github/blob/main/CLA.md

Start by reading callback.mdx and its existing callback endpoint example. Review how the document currently explains proof verification, then expand it with the requested exception, network, timeout, invalid-data, fallback, logging, user-facing error, troubleshooting, and endpoint-security sections. Done means all six task areas are covered with clear examples and solutions.