fleetctl: print out friendly error message from fleetctl list-machines
fleetctl: print out friendly error message from fleetctl list-machines

Print out friendly error message when fleetctl list-machines fails.
Indicate that the API isn't working correctly instead of being
unavailable. Also mention the most likely culprit, which is something to
do with etcd.

Suggested by @robszumski
Fixes https://github.com/coreos/fleet/issues/1203

endocode/fleet,dalbani/fleet,coreos/fleet,jonboulle/fleet,endocode/fleet,dalbani/fleet,jonboulle/fleet,dongsupark/fleet,dongsupark/fleet,giantswarm/fleet,coreos/fleet,giantswarm/fleet,endocode/fleet,dongsupark/fleet,coreos/fleet,giantswarm/fleet,jonboulle/fleet,dalbani/fleet