This method abstracts away the use of `-[SLDevice captureScreenshot:inRect:]`, so I don't think it should leak the requirement of that method that _rect_ be non-null. Rather, if _rect_ is null, let's just have the method log a warning (`-[SLLogger logWarning:]`) like "Could not take screenshot with filename _filename_: Could not determine element's position on-screen."

Since `-rect` requires that the element be valid, though, we should include the standard `SLUIAElementInvalidException` warning here.