Return errors instead of using log.Printf in library code

[derberni]

At a couple of places, the current implementation does not return an error when something fails, but instead does something like the following (taken from systray_windows.go, MenuItem.SetIcon)

	if err != nil {
		log.Printf("systray error: unable to write icon data to temp file: %s\n", err)
		return
	}

However, we would like to handle the error in our own application (and not log it at all). One might even argue, that a library should not write any log messages at all, and only return errors to the main application.

Any chance of a PR changing this behavior getting accepted? After all, it would break backwards compatibility because the function signatures would change from e.g. SetIcon(iconBytes []byte) to SetIcon(iconBytes []byte) error