jsony: fix error message

Originally, we used jsony as a second pass in `configlet lint` after verifying the types. So a JSON parsing error in that pass would mean that configlet had a bug in its type definitions.

However, we now use jsony elsewhere. And we want to share parsing procs where possible (fmt/info/lint/sync). But somebody who currently runs e.g. `configlet info` with a type error in their track `config.json` file will see the weird error message:

https://github.com/exercism/configlet/blob/85e262cdb30bc5b8e37f3321f8031aa592503241/src/helpers.nim#L98-L113

Follow-up from https://github.com/exercism/configlet/pull/564#pullrequestreview-961131591