EBookBackendKolab: error reporting improvements
EBookBackendKolab: error reporting improvements

* use the error mapping utility function to map (KolabMailAccess)
  GError codes to Evolution error numbers for most of the
  backend API functions returning EBookBackendSyncStatus
* leads to better Evolution error messages in case of
  backend errors

GNOME/evolution-kolab,GNOME/evolution-kolab