docs: Add a comprehensive example for handling function tool errors.

[MurtazaAliCode]

This Pull Request adds a comprehensive and runnable code example to the "Handling errors in function tools" documentation section. The goal is to make a complex topic more accessible to new users.

The current documentation accurately describes the theory behind using failure_error_function, but it lacks a practical, complete example that shows the concept in action. This can make it challenging for developers to correctly implement custom error handling.

This PR addresses that gap by providing a clean, working example that demonstrates the following:

• Custom Error Handling in Practice: It shows how to define and use a my_custom_error_function to gracefully handle exceptions within a function tool.
• Correct Implementation: It fixes an issue where the model variable was undefined, ensuring the code is fully runnable out-of-the-box.
• Clear Output: The example correctly shows how the custom error message is returned as the final output, proving that the error was handled as intended without crashing the program.

This contribution makes the documentation more robust and user-friendly, which will significantly improve the developer experience for the community.

[seratch]

I don't think this method signature is correct.

[seratch]

To keep the document page as simple as possible, let's have only the tool and its error function in this code snippet.

[seratch]

I feel "risky" is not the case here. It should be more of something that can fail. How about using an example to perform a backend API call (no need to use real backend; just pseudo code)?