Don't accept ONNX operators that aren't actually ONNX operators

[ezyang]

Right now, if you feed onnx-caffe2 something that happens to be a valid Caffe2 operator, but not an ONNX operator, onnx-caffe2 will silently accept it. That's bad! We do not want to reward bad behavior. And with the versioning update onnx/onnx#186 we can backdoor Caffe2 ops using a vendor extension, if you absolutely need to. So... tighten this up!