Improve `onError` handler

[dlongley]

Currently, we do not log the non-public details of an error on the server -- because we expose all error messages and error names (regardless of public status) as public errors, but we omit details if they aren't marked public. This means that the low-level logger won't log non-public error details because they are dropped. We need a solution here that won't leak non-public messages and error names -- and will properly log non-public error details.