'An unknown error occurred.' when trying to add new custom widget
I feel quite embarrassed, but I have finally got this thing working. The error that I had made was within AdLister_AluWidget.php. I had written at line 4: class Adlister_AluWidget extends BaseWidget instead of what it should have been: class AdLister_AluWidget extends BaseWidget. So, AdLister should have been written with an UPPER-CASE 'L' and not lower-case. If anyone else is struggling, hopefully my error will help them.

The correct code in full:

&lt;?php
namespace Craft;

class AdLister_AluWidget extends BaseWidget
{
    public function getName()
    {
        return Craft::t('Adlister Update');
    }

    public function getBodyHtml()
    {
        return craft()-&gt;templates-&gt;render('adlister/AluWidget');
    }
}