change unwrap to map_err in hyperdrive python bindings

[dpaiton]

The best example is in calculate_targeted_long_with_budget, where we do:

            .map_err(|err| {
                PyErr::new::<PyValueError, _>(format!(
                    "Calculate_targeted_long_with_budget returned the error: {:?}",
                    err
                ))
            })?;

But that's the only place we do it.

In calculate_spot_price_after_short we do something like that but throw away the actual err. In most other fns we use unwrap(), which throws a cryptic panic message. We should follow the targeted long example everywhere.