Maybe some sort of non-modal box could popup informing the user that a filter's
actions have failed to run. Clicking on this would give the user options to
open the filter and correct the issue.
e.g. when you rename a folder that the filter was moving mail to, it can no
longer move mail to that location and should error out.
In my dreams, renaming folder causes automatically update of all dependent
rules. It is quite easy from programmer's point of view. Folder's deletion
should be denied (with precise message) as long as dependent rules exists.
It is much better to heal cause instead of cope with effects.