Add Overflow to DispatchError

[shawntabrizi]

Almost any pallet dealing with numbers will be checking at some point for overflows / underflows in their math.

Each pallet that wants to do that will need to write a custom Error into their pallet for that.

I think Overflow is generic enough that it could fit into the DispatchError enum, and thus be accessible to all pallets without any extra code.

To complete this issue, all existing uses of custom Overflow or Underflow errors should be replaced with DispatchError::Overflow

[shawntabrizi]

This would be a great first issue to anyone new to Rust in general. Try to keep your hands off if you too think this would be easy for you :)

[shaunxw]

If overflow/underflow has an static str, it would be easier to tell which part could overflow. It's helpful when a dispatchable call has multiple overflow/underflow checks for different data.

I think a DispatchError::Arithmetic(ArithmeticError) might be more generic, to handle all common calculation errors, including overflow/underflow, division by zero etc.