Added custom error if error_get_last() does not return anything.

The set_error_handler function in the Application __construct method eats errors from file_get_contents. In this case box shows only a "[RuntimeException]" without any further hint and terminates. It was very frustrating to not see that I had a wrong path to my main-file in the box.json :-(.

Sorry that there is no test case for that in the PR. Phpunit has a custom error handler too and does not let you define own handlers. However without overwriting the handler there is no error for that I could test.