Make sure error object is defined before access its message

[reedfeng]

https://github.com/caolan/async/blob/master/lib/asyncify.js#L90

function handlePromise(promise, callback) {
    return promise.then(value => {
        invokeCallback(callback, null, value);
    }, err => {
        invokeCallback(callback, err.message ? err : new Error(err));
    });
}

err && err.message will be better when I happened to reject a promise without reason.

[aearly]

A PR to fix this would be welcome!

[reedfeng]

@aearly, PR #1569 is created.