Use self.show_errors always

Traceback (most recent call last):
  File "/usr/share/gameconqueror/GameConqueror.py", line 558, in cheatlist_popup_cb
    self.browse_memory(int(addr,16))
  File "/usr/share/gameconqueror/GameConqueror.py", line 705, in browse_memory
    show_error(_('Cannot retieve memory maps of that process, maybe it has exited (crashed), or you don\'t have enough privilege'))
NameError: global name 'show_error' is not defined

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>