"actions menu" step overrides in boost cause fatal errors.

Steps to reproduce:

1. Put the attached file broken.feature in /lib/tests/behat
2. Run behat init
3. Run broken.feature
   Expected: The first test fails as it's trying to use an action menu that isn't present on the page. The second test then runs.
   Actual: The first test crashes with a fatal error. The second test is not run.

The problem is that the step gets the menu node using Element->find() not behat_base->find(), so the step isn't throwing an exception when the menu node isn't found. The fix checks $menunode before trying to call method on it and throws an exception.