Make NewObject return an error

* make it return an error
  * make a canonical error fs.ErrorNotFound
  * make a test for it
  * remove logs/debugs of error

fs/fs.go

@@ -39,6 +39,7 @@ var (
 	ErrorDirExists            = errors.New("can't copy directory - destination already exists")
 	ErrorCantSetModTime       = errors.New("can't set modified time")
 	ErrorDirNotFound          = errors.New("directory not found")
+	ErrorObjectNotFound       = errors.New("object not found")
 	ErrorLevelNotSupported    = errors.New("level value not supported")
 	ErrorListAborted          = errors.New("list aborted")
 	ErrorListOnlyRoot         = errors.New("can only list from root")
@@ -117,8 +118,9 @@ type Fs interface {
 	Info
 	ListFser
 
-	// NewObject finds the Object at remote.  Returns nil if can't be found
-	NewObject(remote string) Object
+	// NewObject finds the Object at remote.  If it can't be found
+	// it returns the error ErrorObjectNotFound.
+	NewObject(remote string) (Object, error)
 
 	// Put in to the remote path with the modTime given of the given size
 	//