PriceImpact error messaging improvement

[johngrantuk]

Common error for PI is occuring when pools have low liquidity - Price Impact calculation for Add Liquidity Unbalanced relies on swap queries under the hood and if they are too large compared to pool balances, they will revert due to MaxAmountInRatio.
See #373 as an example: "Reduce add liquidity amount as otherwise the priceImpact calculation (swap queries) will revert with the pool used, due to revert MaxAmountInRatio"

• Can we catch and return a better error for this case?
• Can we add some docs to explain whats happening so we don't keep revisiting