Standardize error message field in structured logs

What does this MR do and why?

Some of our log messages emitted error as a string, and others emitted error.message. In Elasticsearch, if the error.message is mapping is used, error is an object capable of holding other fields. As a result of these mixed types, Elasticsearch would flag a mapping conflict and drop certain log messages, depending on which value arrived first for the day.

To avoid this mapping conflict, we now standardize on error.message.

Relates to #344015 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.