NPE catched, command class added, beter coding practice applied:

NPE catched, command class added, beter coding practice applied:
- New command class WebProblemCommand added to handle page=problem&problem=???
- Command class WebConfirmCommand redirect to a configurable URL if the user
  instance is not found in registry. This might happen e.g. if the database is
  corrupted or the underlaying file (in LocalFileDatabase class) has been
  removed.
- All user classes inherited from BaseUser class do now set their result
  instance a more clean state (might fix later troubles)