better handle "No space left on device" use case

Currently, when there is not enough space left a message is inserted into the logs in ./nuxeo-drive-client/nxdrive/engine/engine.py:

# IOError: [Errno 28] No space left on device
                 if e.errno == 28:
                     self.noSpaceLeftOnDevice.emit()
                 raise

and in ./nuxeo-drive-client/nxdrive/wui/application.py:

    @QtCore.pyqtSlot()
    def _no_space_left(self):
        dialog = WebModal(self, Translator.get("NO_SPACE_LEFT_ON_DEVICE"))
        dialog.add_button("OK", Translator.get("OK"))
        dialog.exec_()

But nothing is shown to the customer except the following:
https://jira.nuxeo.com/secure/attachment/56426/ErrorMessage_NoDiskFullInformation.png

Desktop Sync continues tries to download file, fails due to lack of disk space.

If should stop and display a message to the user.