Improve error reporting of splay.rpc module

[vschiavoni]

Calling a remote function declared as local reports the error "invalid function name..".
But, the name IS valid: but what it is invalid is the attempt to use it.

Example:

function visible()
    print"i am visible"
end
local function invisible()
    print"i am invisible"
end
procedure={"visible","invisible"}
local f, err = loadstring("return "..procedure[1], "call")
f=f()()
local f, err = loadstring("return "..procedure[2], "call")
print(type(f), err)
f=f()()