Revert "Make jsonrpc-shutdown a noop if process isn't running"

Revert "Make jsonrpc-shutdown a noop if process isn't running"

This reverts commit c580443325a3d071625185876a8f28e04793c625.  It
leads to situations where the sentinel hasn't run yet, which brings
problems if the normal process isn't running, but the stderr
pseudo-process still is.

* lisp/jsonrpc.el (jsonrpc-shutdown): Always enter loop.