Improve error message when missing errors field in rule-tester

[jfmengels]

In version 1.10.3 (and probably earlier), when I forget to specify the errors field in an invalid case when using rule-tester, I get the error 'Cannot read property 'length' of undefined'. I think it would be nice to give a more explicit error message.

ruleTester.run("no-eval", require("../../fixtures/testers/rule-tester/no-eval"), {
    valid: [
        "Eval(foo)"
    ],
    invalid: [
        { code: "eval(foo)" } // Missing `errors` field
    ]
});

I'm willing to submit a PR (actually already prepared one, to update when accepted).

[gyandeeps]

Yes, It happened to me also this morning. I think it would be good to have an appropriate message.

[alberto]

+1

[ilyavolodin]

Sounds good to me! Accepted