Thanks to @netinetwalker for spotting the error, proposing a fix, and testing it.
Error due to @est31's merging changes to PR #3202 to add more error reporting for invalid reply types, commit:
524a7656e3 "redis: throw error if block request failed"
Now we branch out on the valid reply type "not found".
Fixes#3196. Before, we didn't throw an error, and the engine thought the
block isn't occupied. But in fact it might be that redis is still loading,
and the block does exist in the database. The result was a cheesy map.
