fix foxway exceptions, now showing error messages for users

* use '$return[] = (string) new ExceptionFoxway' instead of 'throw'
* use 'array_key_exists' instead of 'isset' for checking variable definition

Time: 478 ms, Memory: 24.50Mb
OK (478 tests, 484 assertions)

Change-Id: I3f59306c498205777a4dc04127ba4b6f0e471265