Correct issues with remote indexing and catch incorrect calls for LuaRPC

helper_remote_index() contained an error which allowed for incorrect
calculation of the string length to be sent. This should now be
fixed. Additionally, read_cmd_call() did not check whether indexing
was successful at each step, we now check whether the indexed result
is callable or a table and attempt to provide an indication of the
type of error made by the user.