Try to report source of bad Lua API calls

Okay. Sorry for spamming up the issues a little, I haven't contributed anything on github for a long time. This pull request will attempt to find the source of bad calls to the Redis Lua API, and should address issue #1121.

For instance, here is an example of it correctly reporting the source line of an error.

> redis 127.0.0.1:6379> eval "\n\n\n\nredis.call(\"del\")" 0
> (error) ERR Error running script (call to f_02e4b6c5f49751ef9096706ae82901bb51bf59f2): @user_script: 5: Wrong number of args calling Redis command From Lua script