Correction to callback invokation.

`callbackWithError` was allegedly expected to call `callback` with any arguments beyond `msg` and `callback`. The code being corrected was calling `Array.prototype.slice(...)` which attempts to slice `Array.prototype` and not `slice`'s first argument (`arguments`). I think the intention was to call `Array.prototype.slice` in the context of `arguments`, passing `2` as the 1st parameter.

I'm not sure how I feel about this error handling pattern in general, but I'll look for a more appropriate place to voice my opinion on that. :)