For unknown reasons, the catch failed on Mac. So I am switching
to the lower level API which avoids exceptions and is used
elsewhere in VTK. At same time I cleaned up the delete.

notes about the old approach:
catch std::exception (json::RuntimeErrror's grandparent) succeeds
typeid(e).name is vtkjson::RunttimeError (mangled)
typeid(e) == typeid(a manufactures json::RuntimeError) succeeds
dynamic_cast<json::RuntimeError> fails