structured logging of error sources in json formatter

[radix]

As @hawkw mentioned in #1347 (comment) - it should hopefully be possible to get the JSON formatter to implement record_error to log sources of errors in a structured way. I tried to take a crack at this in the 0.1.x branch and I'm not having luck.

Here's the patch I'm trying: https://gist.github.com/radix/8a673a4c36512560a1f132dbf634508e

The problem is that record_error is not being called at all. I figured that by passing &error as &(dyn std::error::Error + 'static) should force use of the correct implementation of Value, but it's just not being called.