Reduce log level of pub_worker proxy-related failures.

[isoos]

• Fixes Improve retries when uploading worker results? #8969 and Look into failing workers #8338.
• Reduces log level of proxy-related failures, since we usually cannot do much about it.
• Investigated the worker's retry logic and it is already working with larger retry window (retrying after 10-20-40 seconds), and I don't think increasing that is important at this point, but we could do that too.