Add optional error handler to queues

[jnordberg]

Allowing you to implement things like error logging and task retries more easily, like so:

var q = async.queue(function(task, callback) {
    console.log('Oops.');
    callback(new Error('I fail every time'));
});

q.error = function(error, task) {
    if (task.retries == null) task.retries = 0;
    task.retries++;
    if (task.retries < 3) {
        q.push(task);
    } else {
        console.log('This isnt working...');
    }
};

q.push({foo: 'bar'});

[aearly]

Looks good. This feature has been requested a few times before.

Can you add a note to the doc block about the feature?

[jnordberg]

Sure, to the README right?

[megawac]

@jnordberg to the jsdoc above the function preferably. The readme docs are on their way out