fleetctl: improve getUnitFile() error handling and add documentation
fleetctl: improve getUnitFile() error handling and add documentation

jonboulle/fleet,giantswarm/fleet,coreos/fleet,dalbani/fleet,endocode/fleet,jonboulle/fleet,improbable-io/fleet,giantswarm/fleet,dongsupark/fleet,dalbani/fleet,coreos/fleet,dongsupark/fleet,jonboulle/fleet,giantswarm/fleet,coreos/fleet,improbable-io/fleet,endocode/fleet,endocode/fleet,dalbani/fleet,improbable-io/fleet,dongsupark/fleet