chore: exposing ErrCode class

[iuricmp]

Expose ErrCode class and add rpc_pb to exports.
Useful for client who wants to implements error checks on their dApps, eg:

import { ErrCode } from "@gnolang/gnonative";  

if (err.errCode() === ErrCode.ErrDecryptionFailed) {
  setError("Wrong password, please try again.");
}

[jefft0]

LGTM. I assume this will be tested in dSocial.

[D4ryl00]

tested in Android and iOS with this code in a catch block:

try {
...
} catch (error) {
	if (error instanceof ConnectError) {
	  const err = new GRPCError(error);
	  if (err.errCode() === ErrCode.ErrBridgeInterrupted) {
	    console.log('err catched');
	  }
	} else {
	  console.log(error);
	}
}