Frontend Ajax: Check if module exists

This solution is based on (or copied from...) the language validation above my code.

While reviewing this request, please check:
- There is no need to check the action in FrontendAjax because there is already one later in FrontendAjaxAction.
- There is no need to do an exit after the module validation because that is already done by the output function of FrontendBaseAJAXAction.
- There is no need for an extra fix in BackendAjax because of the authentication check.

Correct?