Match error causes front end commands to hang

[dongahn]

#677 (comment)

When the underlying graph hierarchy doesn't match with a jobspec, our graph infrastructure code returns an match error:

2020-06-24T18:30:21.552622Z sched-fluxion-resource.err[0]: match_request_cb: match failed due to match error (id=0): Success
2020-06-24T18:30:21.552668Z sched-fluxion-resource.err[0]: match_request_cb: flux_respond_error: Invalid argument

Unfortunately, the error is propagated such that the front end commands including flux-ion-resource and flux-mini-run cannot gracefully the error.

[dongahn]

I want to spend a bit more time for this. Targeting Sep release.