local_timezone: gracefully handle retrieval errors on Darwin

[tpwrules]

In some sandbox environments, the program may not have permission to access /etc/localtime or it may not exist.

Catch any exceptions accessing and parsing its value and return UTC by default to avoid crashing the program.

Pull Request Check List

• [ ] Added tests for changed code.
• [ ] Updated documentation for changed code.

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #792 will not alter performance

_{Comparing tpwrules:darwin-error-handling (a4d98ce) with master (3e3fec6)}

Summary

✅ 1 untouched benchmarks

[tpwrules]

Fixed formatting.