The json parsing library gives us an error path where the parse
failed, which means we can index into the path to show the json value
that failed us.
This can be quite expensive (and large!) of course, but the error
message clarity is worth it methinks.
Change-Id: Icacbd799254aaecd4a939ca13e6070d68a78138d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12952
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>
