added an error callback to the hidden check of the clear completed items...

added an error callback to the hidden check of the clear completed items button. This was done so that frameworks like mithril who remove elements from DOM in stead of putting css-classes like .hidden on elements get through the test suite

tests/testOperations.js

@@ -30,6 +30,8 @@ function TestOperations(page) {
 	this.assertClearCompleteButtonIsHidden = function () {
 		page.tryGetClearCompleteButton().then(function (element) {
 			testIsHidden(element, 'clear completed items button');
+		}, function (_error) {
+			assert(_error.code === 7, 'error accessing clear completed items button, error: ' + _error.message);
 		});
 	};