Replace Monoid instance for Monoid with combineDepError function
Replace Monoid instance for Monoid with combineDepError function

Given that `mappend` was partial (as intended), a simple function
seemed saner than a surprising Monoid instance.

AndreasPK/stack,Fuuzetsu/stack,AndrewRademacher/stack,juhp/stack,deech/stack,sjakobi/stack,mrkkrp/stack,Blaisorblade/stack,mrkkrp/stack,AndrewRademacher/stack,deech/stack,MichielDerhaeg/stack,Blaisorblade/stack,martin-kolinek/stack,MichielDerhaeg/stack,Heather/stack,anton-dessiatov/stack,sjakobi/stack,juhp/stack,Heather/stack,AndreasPK/stack,anton-dessiatov/stack,martin-kolinek/stack,Fuuzetsu/stack