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/util/index.js

@@ -0,0 +1,16 @@
+'use strict'
+
+const chai = require('chai')
+const expect = chai.expect
+
+const expectErrCode = async (p, code) => {
+  try {
+    await p
+  } catch (err) {
+    expect(err).to.have.property('code', code)
+    return
+  }
+  expect.fail(`Expected error with code ${code} but no error thrown`)
+}
+
+module.exports = { expectErrCode }