Error handling in a size_t hashfunction
I have never seen anyone doing error handling in the calculation of a hash function. 

For a hash table to work, items must be equatable and hashable. If they are not equatable then sorry, you lost. As long as you can detect that an item is “bad” for equatability, you can just make it compare different to any item other than itself. And make it produce a hash code if constant zero or any other constant. 

If you have problems calculating a hash function then you have a bigger problem that you need to solve first.