Fix redis erroring on non found blocks

Fix redis erroring on non found blocks

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:

524a7656e3e5cd671b05c13e2ad69cb84bad0423 "redis: throw error if block request failed"

Now we branch out on the valid reply type "not found".