chore: add error codes (#155)

* chore: add error codes

* chore: create errors with new Error()

* fix: better error testin

* refactor: simplify random bytes error checks

test/random-bytes.spec.js

@@ -0,0 +1,27 @@
+/* eslint-env mocha */
+'use strict'
+
+const chai = require('chai')
+const dirtyChai = require('dirty-chai')
+const expect = chai.expect
+chai.use(dirtyChai)
+
+const randomBytes = require('../src/random-bytes')
+
+describe('randomBytes', () => {
+  it('produces random bytes', () => {
+    expect(randomBytes(16)).to.have.length(16)
+  })
+
+  it('throws if length is 0', () => {
+    expect(() => randomBytes(0)).to.throw(Error).with.property('code', 'ERR_INVALID_LENGTH')
+  })
+
+  it('throws if length is < 0', () => {
+    expect(() => randomBytes(-1)).to.throw(Error).with.property('code', 'ERR_INVALID_LENGTH')
+  })
+
+  it('throws if length is not a number', () => {
+    expect(() => randomBytes('hi')).to.throw(Error).with.property('code', 'ERR_INVALID_LENGTH')
+  })
+})