Allow simpler tests for error.code #166

Support plan

• is this issue currently blocking your project? (yes/no): no
• is this issue affecting a production system? (yes/no): no

Context

• node version: any
• module version: 8.0.3
• environment (e.g. node, browser, native): node
• used with (e.g. hapi application, another framework, standalone, ...):
• any other relevant information:

What problem are you trying to solve?

To test for the .code property attached to node errors (instead of .message) that are not fixed).

Currently I have to do this manually, eg. for throw():

const Code = require('code');
const expect = Code.expect;

const err = expect(() => process.chdir(42)).to.throw(TypeError);
expect(err).to.contain({ code: 'ERR_INVALID_ARG_TYPE' });

// or as one-liner
expect(expect(() => process.chdir(42)).to.throw(TypeError)).to.contain({ code: 'ERR_INVALID_ARG_TYPE' });

Do you have a new or modified API suggestion to solve the problem?

Add a third has parameter to throw() and reject(), which takes an object which is passed to a contain() test. Also make the message optional (works since it uses a string and type must be instanceof Error). Eg.

expect(() => process.chdir(42)).to.throw(TypeError, { code: 'ERR_INVALID_ARG_TYPE' });