Only intercept exceptions of type "NotFoundException" instead of any Exception

The sharing backend may throw another exception for example when the activity app encounters a problem. Previously this just triggered a 404 error page and the exception got not logged at all. With this change such exceptions get not intercepted and regularly handled as exceptions so that we have meaningful log data. Also the user will be shown a window informing him that an error happened.

Helps to debug cases such as https://github.com/owncloud/core/issues/19465