make die reliably hand error to post-eval code

make die reliably hand error to post-eval code

Put the exception into $@ last thing before longjmping to the op following
the eval block, where previously it went into $@ before unwinding the
stack.  This change means that the exception is not liable to be lost
by $@ being clobbered by destructors, cleanup code, or restoration after
"local $@".  The code running immediately after eval can now rely on $@
accurately indicating the exception status of the eval.