Issues with "Check popover validity" in popover target attribute activation behavior

[nt1m]

1. Check popover validity there does not specify whether to throw exceptions or not (I think here it would make more sense if the answer was no).
2. If it doesn't throw exceptions, that check is redundant with the one that the "show popover" or "hide popover" algorithms invoke.

[nt1m]

cc @mfreed7 @josepharhar @annevk @domenic

[josepharhar]

Thanks for catching this. Exceptions should not be thrown, and yeah the checks are redundant so they can be removed entirely.

[annevk]

I'll add a fix to #8962. More review of that PR welcome btw.

[annevk]

Wait, I don't think it's redundant in the hide popover case since presumably we don't want to update the popover invoker? I will fix that instead to ensure it doesn't throw exceptions.