If you happen to use Zend tool to create a project and globally turn off the view rendering in your control logic, the error controller will also neglect to show a stack-trace or error details. Add the following code to the error controller just in case.

public function __construct(Zend_Controller_Request_Abstract $request, Zend_Controller_Response_Abstract $response, array $invokeArgs = array()) {
	parent::__construct($request, $response, $invokeArgs);
	$this->getFrontController ()->setParam ( "noViewRenderer", false );
}

It’s also better to disable the view rendering for specific controller actions and not globally, hence the Zend Framework documentation explains it best:

Class FooController extends Zend_Controller_Action {
	public function init() {
		// Local to this controller only; affects all actions,
		// as loaded in init:
		$this->_helper->viewRenderer->setNoRender ( true );

		// Globally:
		$this->_helper->removeHelper ( 'viewRenderer' );

		// Also globally, but would need to be in conjunction with the
		// local version in order to propagate for this controller:
		Zend_Controller_Front::getInstance ()->setParam ( 'noViewRenderer', true );
	}
}

More detailed information at:
Action Controllers