Add nested json validation

[JackWilb]

This is left over from my big validation push in pr multinet-app/multinet-server#141

[waxlamp]

@AlmightyYakob, this is done right?

[jjnesbitt]

@AlmightyYakob, this is done right?

I'm actually not sure. Currently it doesn't look like we raise anything that inherits from ValidationFailure during any of the nested_json upload.

[waxlamp]

Would you mind looking into it?

[jjnesbitt]

Sure thing