flux-run: poor error message on bad input

[chu11]

Recently had a typo in a test script that took me awhile to chase down.

>flux run --dry-run --setattr=exec.test.run_duration=0.1s "hostname"

works of course, but I screwed up quoting and accidentally did

>flux run --dry-run "--setattr=exec.test.run_duration=0.1s hostname" 
flux-run: ERROR: job command and arguments are missing

obviously job command isn't missing. I assume some parsing issue somewhere leading to this less than helpful error message

[grondo]

My guess is you're quoting of the entire --setattr option caused argparse to treat exec.test.run_duration=0.1s hostname as its entire argument, thus there was no "remainder" to represent the command and arguments.

I'm not sure we'll be able to fix this one.