I investigated, and it turns out that CouchDB gives very weird and even inconsistent results when there's an error in the reduce function.  I think the safest thing to do is to just set `e.value` to `null` and keep going.  You'll at least be consistent with what I occasionally see CouchDB do.

If you're curious, here's an example of a test that returns inconsistent results.  Occasionally CouchDB reduces the keys to 3 values, occasionally it returns all 4: https://gist.github.com/nolanlawson/10339055.