[native_rust_library] fix display text for error enum

Summary: small change to improve the error strings we send back to JS

Test Plan: console logged the TonicGRPC error variant in JS and confirmed it only contained the gRPC message and no extraneous info

Reviewers: will

Reviewed By: will

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10669