[swaps] Improve error handling in zrml-swaps

[maltekliemann]

The swaps pallet currently raises some questionable errors, for example MathApproximation. These seem very uninformative.

For example, if I call pool_join with a small amount of liquidity (in comparison to what's already in the pool), then ratio ends up being zero, and I get a MathApproximation error:

zeitgeist/zrml/swaps/src/utils.rs

Lines 105 to 108 in b00d69f

	let ratio: BalanceOf<T> =
	bdiv(p.pool_amount.saturated_into(), total_issuance.saturated_into())?.saturated_into();
	Pallet::<T>::check_provided_values_len_must_equal_assets_len(&p.pool.assets, &p.asset_bounds)?;
	ensure!(ratio != Zero::zero(), Error::<T>::MathApproximation);

How am I supposed to know what happened?