MDL-58948 behat: catch ElementNotFoundException

Another valid exception that can be thrown from a Mink driver
is the ElementNotFoundException when an element has gone missing.

lib/tests/behat/behat_general.php

@@ -634,6 +634,9 @@ class behat_general extends behat_base {
                     } catch (WebDriver\Exception\NoSuchElement $e) {
                         // Do nothing just return, as element is no more on page.
                         return true;
+                    } catch (ElementNotFoundException $e) {
+                        // Do nothing just return, as element is no more on page.
+                        return true;
                     }
                 }