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 discoverd by running go tools vet.

Author: Dongsu Park

fleetctl/fleetctl.go

@@ -833,18 +833,19 @@ func lazyCreateUnits(cCmd *cobra.Command, args []string) error {
 // Returns true if the contents of the file matches the unit one, false
 // otherwise; and any error encountered.
 func matchLocalFileAndUnit(file string, su *schema.Unit) (bool, error) {
-	result := false
 	a := schema.MapSchemaUnitOptionsToUnitFile(su.Options)
 
 	_, err := os.Stat(file)
-	if err == nil {
-		b, err := getUnitFromFile(file)
-		if err == nil {
-			result = unit.MatchUnitFiles(a, b)
-		}
+	if err != nil {
+		return false, err
+	}
+
+	b, err := getUnitFromFile(file)
+	if err != nil {
+		return false, err
 	}
 
-	return result, err
+	return unit.MatchUnitFiles(a, b), nil
 }
 
 // isLocalUnitDifferent compares a Unit on the file system with a one