Improve error message when metamethod causes error, blaming the caller

Without this modification, the error message looked like this:

127.0.0.1:6379> EVAL "local a = redis.blurbs('juarez'); return a" 0
(error) ERR Error running script (call to f_0711c332f701b77d14dcf547568347798053f6d6): @metatable_def:4: @metatable_def: 4: Unknown Redis command called from Lua script

Now the @user_script and the line are referenced:

127.0.0.1:6379> EVAL "local a = redis.blurbs('juarez'); return a" 0
(error) ERR Error running script (call to f_0711c332f701b77d14dcf547568347798053f6d6): @user_script:1: @metatable_def: 4: Unknown Redis command called from Lua script