Raises an exception when trying to connect device with zero notes.

[a]

When connecting a device which has zero notes stored in it, following traceback gets printed:

Traceback (most recent call last):
  File "/usr/bin/m210c-qt", line 145, in list_selection_changed
    self.update_group_box()
  File "/usr/bin/m210c-qt", line 195, in update_group_box
    self.dataview.set_data(data)
  File "/usr/lib/pymodules/python2.6/m210/qt/dataview.py", line 52, in set_data
    self.__notes = list(m210.pegatech.note_iter_from_data(data))
  File "/usr/lib/pymodules/python2.6/m210/pegatech.py", line 421, in note_iter_from_file
    state, note_num, note_max_num = struct.unpack("BBB", notes_file_read(3))
struct.error: unpack requires a string argument of length 3
ERROR:dbus.connection:Exception in handler for D-Bus signal:
Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.6/dbus/connection.py", line 214, in maybe_handle_message
    self._handler(*args, **kwargs)
  File "/usr/bin/m210c-qt", line 239, in dbusev_device_managed_changed
    self.update_group_box()
  File "/usr/bin/m210c-qt", line 195, in update_group_box
    self.dataview.set_data(data)
  File "/usr/lib/pymodules/python2.6/m210/qt/dataview.py", line 52, in set_data
    self.__notes = list(m210.pegatech.note_iter_from_data(data))
  File "/usr/lib/pymodules/python2.6/m210/pegatech.py", line 421, in note_iter_from_file
    state, note_num, note_max_num = struct.unpack("BBB", notes_file_read(3))
error: unpack requires a string argument of length 3