[Feature idea] Mutual exclusive return signatures

[sewbacca]

Especially usefull for save function calls, returning nil, err on failure.

If I add logic, the issue just wanders of, like an entangled particle.

So the solution is to @cast err to string, but I wonder if maybe a mutual exclusive return type is more appropriate. It could solve this issue and others too.
I dunno what a convinient syntax would be, something along those lines:

---@param path string
---@param mode string
---@return file* | (nil, string)
function io.open(path, mode) end