cache more set/get exception in backend

See #57 :

> I saw that you catched get/set error [here](https://github.com/thadeusb/flask-cache/blob/master/flask_cache/__init__.py#L221) and [here](https://github.com/thadeusb/flask-cache/blob/master/flask_cache/__init__.py#L425) , but if set/get fail there is a big chance that `cache_key = decorated_function.make_cache_key(*args, **kwargs)` fails 3 line above in each case and also [here](https://github.com/thadeusb/flask-cache/blob/master/flask_cache/__init__.py#L531) .

> should I move it inside the try/catch too ? and wrap in a try catch for the 3rd case ?