bring G_KEEPERR back to the realm of sanity

bring G_KEEPERR back to the realm of sanity

Makes the G_KEEPERR logic more consistent, and in particular make it
sensibly handle non-string exceptions.  An exception in a destructor
is now always emitted as a warning, and never copied or merged into
$@ of the surrounding context.  No more clobbering exceptions being
handled elsewhere, and no more double reporting.  This fixes the rest of
[perl #74538].