Describe the bug

Discovered when testing #182. The following script:

local conj_ttl = redis.call('ttl', conj_key)
if conj_ttl < timeout then
    -- some code
end

expects conj_ttl always to be a number. Dragonfly instead returns errors as lua tables (aka js-like objects) with a detailed cause.

To Reproduce

Dragonfly:

127.0.0.1:6379> EVAL "return redis.call('ttl', 'nonexistent')" 0
(error) ERR script tried accessing undeclared key

Redis:

127.0.0.1:6379> EVAL "return redis.call('ttl', 'nonexistent')" 0
(integer) -2

Expected behavior

Return sentinel errors in scripts as lua numbers