lua: slightly nicer diagnostics output

When Lua itself prints errors such as:

  Error: [string "mp.defaults"]:387: syntax error near 'function'

It's unclear why the location is prefixed with "string ". And for some
reason, it disappears if you prefix the name with '@'. I suppose this is
done for the sake of luaL_loadstring. With the '@' prefix added, the
output is now:

  Error: mp.defaults:387: syntax error near 'function'