There are some problems in the way filters handle validation errors.

1. Filters don't save input values if a validation error occurs. For example, if you go to configuration -> items, set "Type" to some value, set "Keep history" to 999999 and press "Filter" an error message will be displayed and both filter fields will be reset. I think we should display all of the values just as we do in other forms.
2. Some filter parameters are marked as P_SYS. This means, that passing an incorrect value will cause a fatal error and will not display the page at all.
3. We should decide if we need to try to display the requested data if the submitted filter is invalid. For instance, if we submit an incorrect filter on the item configuration page, we still display a list of items.