Decide how to handle arithmetic errors (e.g. division by 0)

[pbiggar]

some arithmetic operations can lead to errors:

• division by 0
• modulus by 0

These should return Result (Int, IntError):

Int::divide(Int: a, Int: b) -> Result (Int, Error)
Int::mod(Int: a, Int: b) -> Result (Int, Error)
Int::sum(List Int: a) -> Int

One problem here is that the error rail is a little cumbersome, and so making obvious int operations use Results will make them unweildy to use. So we need to iterate on the error rail before this makes sense, practically.

[StachuDotNet]

folding this into #5243