Improve error messages and logging in Charon when processing validator exits

[boulder225]

🎯 Problem to be solved

When processing validator exits, Charon's error messages and logging do not provide enough detail to easily identify the specific causes of failures. This makes it difficult for developers to diagnose and resolve issues.

🛠️ Proposed solution

• [ ] Review the current error handling and logging code paths related to processing validator exits
• [ ] Update error messages to include relevant details such as validator IDs, error codes, or other contextual info
• [ ] Add logging statements to capture important state or inputs/outputs at different steps of the exit process
• [ ] Test changes by intentionally triggering various exit failure scenario
• [ ] Update relevant documentation or runbooks