pkg/asset/store: Use '%q' for formatting quoted strings

This saves us a few characters and gives us better handling when the
string values themselves contain quotes (although only the former
matters much in this case).  The old manual quoting is from 971eea9
(pkg/asset: Save/Load functinality for assets into a state file,
2018-10-11, #388).

pkg/asset/store.go

@@ -113,7 +113,7 @@ func (s *StoreImpl) fetch(asset Asset, indent string) error {
 	logrus.Debugf("%sLooking up asset from state file: %s", indent, reflect.TypeOf(asset).String())
 	ok, err := s.GetStateAsset(asset)
 	if err != nil {
-		return errors.Wrapf(err, "failed to unmarshal asset '%s' from state file '%s'", asset.Name(), stateFileName)
+		return errors.Wrapf(err, "failed to unmarshal asset %q from state file %q", asset.Name(), stateFileName)
 	}
 	if ok {
 		logrus.Debugf("%sAsset found in state file", indent)