I think you're trying to solve the wrong problem. You should never get that unrar problem. Sounds like Newsbin thinks the rars have landed when they're still not on disk or something.
([21:20:48] ERROR UnRAR Error:Volume open error).
Anyway, Sickbeard has no way to ID failures. When I first wrote the integration, because of how the API works, I expected SB to be able to but it's simply not coded that way. On the other hand Sonarr talks using the API to newsbin and both tracks the download and when a file set fails.
In the options there's a URL defined. That's how Newsbin tells SB that a download succeeds. There's no way to tell SB that a download failed though.