die_unwind(): mortalise, not mortalcopy the err SV

die_unwind(): mortalise, not mortalcopy the err SV

The error string needs to be preserved while unwinding the stacks,
but doing a simple sv_2mortal() and bumping the reference count seems
sufficient, rather than making a complete copy.

Also, avoid the mortalised SV's buffer from being stolen by using the
SV_NOSTEAL flag rather than unsetting SvTEMP.

Finally, add some basic comments above Perl_die_unwind() explaining what
it's for.