While running with --jsonnet-implementation binary:/usr/local/bin/jrsonnet, I got an error:

Error: invalid character 'T' looking for beginning of value

json.Unmarshal returns this cryptic message, meaning the input was not valid JSON.

I use std.trace in my code and Tanka was passing both stdout and stderr from jrsonnet. The latter causing the unmarshalling error.

This PR captures the stderr output separately now, passing it along to zerolog instead.