fix Runner#fail logic to seamlessly support Error-like objects. Closes #1677

[igorraush]

No description provided.

[boneskull]

this will throw an exception if err happens to be null. add additional checks.

I would also probably avoid creating the variables if we are not going to use them elsewhere.

[igorraush]

@boneskull Sorry, I fail to see how it will throw an exception when err is null :( This test passes for me:

it('should emit a helpful message when failed with null', function(done){
  var test = {}, err = null;
  runner.on('fail', function(test,err){
    err.message.should.equal('the null [null] was thrown, throw an Error :)');
    done();
  });
  runner.fail(test, err);
})

The message could definitely be more helpful, but no crash.

[boneskull]

sorry, n/m, I think I was reading the example in the original issue instead