Previously, 'quit' could cause remote endpoints to never get a
reply and thus sometimes hang. Ensure we always reply. Also,
give the application a chance to signal jsonrpc-error with the
served code=32000, meaning "no error".
* doc/lispref/text.texi (JSONRPC Overview): Rework section on
request dispatchers.
* lisp/jsonrpc.el (jsonrpc-connection-receive): Rework.
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