Remove ofono_error messages for not found files

This MR removes ofono_error messages  and changes them to DBG() messages for not found files. Additionally, it makes changes and fixes bugs that appeared when trying to do this:

* gril now returns a NULL pointer in message->buf instead of rubbish when there is no event data.
* Fix a problem with maguro: no active calls were being returned as an empty event data
* Fix a non-related bug: no data connection was being established in maguro due to not having a default value for the maximum number of possible active calls