fix up WriteJson for DAP error handling

[TylerLeonhardt]

Now in my handler I can do:

throw new RpcErrorException(0, "A positive integer must be specified for the processId field.");

and then I get:

One thing that does concern me is that the DAP spec says that command is required in Response bodies, but this functionality works as I want so I'm going to just ignore that detail for now as it might actually be optional.