When using regular grpc client on error we get ServiceError with all metadata assigned as attributes:
https://github.com/grpc/grpc-node/blob/master/packages/grpc-js/src/call.ts#L79

In fallback client we are only getting Error with JSON string as message:
https://github.com/googleapis/gax-nodejs/blob/master/src/fallback.ts#L337

To be able to handle different errors differently it would be convenient to use similar structure i.e. to make error.code available without need of parsing message.