fix: handle messages with custom error code in error_reply

[pkierski]

Redis treats first "word" of error message as custom error code, So redis.error_reply("foo bar") returns "-foo bar" but not "-ERR foo bar".
This is a problem for testing application which relies on exact error message returned by Lua script.

[alicebob]

thanks for the PR, looks good. So many werid edge cases in redis :)
Can you squash the commits? If I do that you won't get credits.

[pkierski]

Squashed.

[alicebob]

great, thanks for the fix. Will make it a proper version in a few days.