Name JSON key when erroring

When running `jtd-codegen`, I get an error like:

```console
thread 'main' panicked at 'no entry found for key', /project/crates/core/src/codegen/mod.rs:123:44
```

But there's no hint of what "key" was being used at the time of the error.

For this error, can you provide a hint as to which JSON key has no entry?