more descriptive name for EmptyErrror

[Benjmhart]

this should be InvalidJson Provided

[Benjmhart]

actually upon further inspection, this appears to be the error received if a given property name is not present.

again, more descriptiveness is needed.

[kozross]

This is actually more complicated - it's an artifact of how we combine non-determinism and error handling. It definitely needs fixing, and should also make the whole codebase cleaner to boot.