The record will not be edited if there is an error (either validation rule or trigger).
There is no way to handle a time-based workflow field update failure currently.
So if you want feedback on the failure of the update, I see only 2 options:
- create a report that you run periodically to check for records that have an invalid status
- do critical updates via apex triggers, which do allow you to catch and process failed updates
WRT 2, you could either use the time-based workflow to schedule an update to the record (only updating a non-validated field) and then base a trigger on that to do the actual update, or use a batch job (e.g. nightly) to identify and update the relevant records.
FYI some earlier discussion on this subject here: Bests practices for handling potential errors as a result of time based workflow