shell: error from input plugin when upstream broker dies could be improved

[grondo]

Flux was accidentally shut down on a node with a job still running, and the first error reported was:

flux-shell[1050]: FATAL: input.kvs: flux_job_event_watch_get: Function not implemented

(this message is also recorded as a job exception)

In this case, we can deduce that the upstream broker has gone away, so the error message could be improved to
"lost my parent broker" or similar. (Also I wonder if this should even be a fatal error)