registry,fleetctl: fix bugs regarding shadowed error variables

[dongsupark]

• registry: make CreateUnit() explicitly return error

: When marshal() returns a non-nil error 'err', which shadows the pre-defined 'err', we should explicitly return the second error returned from marshal(). Otherwise it will always return nil. To fix other potential errors, let's just return err explicitly. This was discovered by running "go tool vet":

$ go tool vet --shadow ./registry
registry/job.go:316: declaration of "err" shadows declaration at registry/job.go:315

• fleetctl: fix wrong error value from matchLocalFileAndUnit()

: In matchLocalFileAndUnit(), if os.Stat() returns nil and getUnitFromFile() returns non-nil error, then in the end matchLocalFileAndUnit() will return (false, nil). That's not the expected result. It should have returned the second error that came from getUnitFromFile(). To avoid further potential bugs, let's follow the idiomatic coding style: first check "err != nil", and fail-fast. This issue was discovered by running "go tool vet".

[tixxdz]

lgtm thanks @dongsupark , don't know why or if golang is supposed to warn about a local variable that shadows another local variable.

[dongsupark]

@tixxdz Merged. Thanks for the review!