add option to log 4xx error details in request logs

[jeffmhastings]

Fixes #111

This PR adds a new option log4xxResponseErrors that, when true, will add the error details to the response log under the key err (chosen to match 500 error logging).

For example:

{"level":30,"time":1683229159104,"pid":92186,"hostname":"jeff-MacBook-Pro","req":{"id":"1683229159103:jeff-MacBook-Pro:92186:lh9j6jej:10000","method":"get","url":"/","query":{},"headers":{"user-agent":"shot","host":"jeff-MacBook-Pro:0"},"remoteAddress":"127.0.0.1","remotePort":""},"err":{"type":"Object","message":"invalid request","stack":"","statusCode":400,"error":"Bad Request"},"res":{"statusCode":400,"headers":{"content-type":"application/json; charset=utf-8","cache-control":"no-cache","content-length":68}},"responseTime":1,"msg":"[response] get / 400 (1ms)"}

This pulls the response from the request.response.source object, which is the value returned by the lifecycle method.