* lisp/eshell/esh-io.el (eshell-pipe-broken): New error.
(eshell-output-object-to-target): Signal 'eshell-pipe-broken' if the
target is an exited/signaled process.
* lisp/eshell/esh-proc.el (eshell-insertion-filter): Handle
'eshell-pipe-broken'.
* test/lisp/eshell/esh-proc-tests.el: New test.