fix: improve error handling in ephemeral validator

[reallesee]

Replace expect() with proper error handling using map_err() when writing the ephemeral validator config file. This prevents program panics and provides more informative error messages to users when file operations fail.

Greptile Summary

This PR improves error handling in the ephemeral validator code by replacing a potentially dangerous expect() call with proper error propagation using map_err() when writing the configuration file. The change specifically targets the config file writing operation, making it more robust by providing detailed error messages instead of panicking.

The modified code now properly propagates file system errors with additional context, which is crucial for debugging issues related to file operations (permissions, disk space, etc.). This is a step in the right direction for making the codebase more reliable and maintainable.

However, it's worth noting that there are still several other expect() calls in the same file that could benefit from similar error handling improvements:

• Balance checking operations (2 instances)
• Validator startup
• Validator termination
• Cleanup operation

Confidence score: 4/5

1. This PR is safe to merge as it strictly improves error handling without changing core functionality
2. The score reflects that while this is a clear improvement, similar patterns remain unaddressed in the same file
3. Files needing attention: crates/bolt-cli/src/ephemeral_validator.rs - the remaining expect() calls should be reviewed

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