Fix panic when Lua integer is out of range of destination type

[skk64]

Hey, I found this bug when using toAny with structs that have u8 types. For example, the following code panics:

try l.doString("MyInt = 256");
_ = l.getGlobal("MyInt");
const my_int = try l.toAny(u8, -1); // Panic!

Changing it to return an error means the host code has the option of handling the error and continue running.