object.Tree.FindEntry should return ErrFileNotFound rather than errEntryNotFound

If directory is not found, it will return the public error `ErrDirNotFound`. For a missing file, it returns `errEntryNotFound`: https://github.com/src-d/go-git/blob/f01958913fab6e1967c1317b7222d1160212371c/plumbing/object/tree.go#L178

Since the error is private there isn't a way to programmatically check for that condition.