refactor(gamm): improve error handling and messages when parsing pool…

… assets (#2804)

Closes: #XXX

## What is the purpose of the change

The errors were non-descriptive, and, as a result, making the debugging experience more difficult. This refactor improves error messages.

This is not state breaking because we still return error if any of the denoms are missing. Error messages do not affect state breaks

## Testing and Verifying

This change is a trivial rework / code cleanup without any test coverage.

## Documentation and Release Note

  - Does this pull request introduce a new feature or user-facing behavior changes? no
  - Is a relevant changelog entry added to the `Unreleased` section in `CHANGELOG.md`? yes
  - How is the feature or change documented? not applicable

CHANGELOG.md

@@ -45,8 +45,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Features
 
 * [#2739](https://github.com/osmosis-labs/osmosis/pull/2739) Add pool type query
+
+### Bug fixes
+
 * [#2803](https://github.com/osmosis-labs/osmosis/pull/2803) Fix total pool liquidity CLI query.
 
+### Misc Improvements
+
+* [#2804](https://github.com/osmosis-labs/osmosis/pull/2804) Improve error handling and messages when parsing pool assets.
+
 ## v12.0.0
 
 This release includes several cosmwasm-developer and appchain-ecosystem affecting upgrades:

x/gamm/pool-models/balancer/pool.go

@@ -248,8 +248,12 @@ func (p Pool) parsePoolAssetsByDenoms(tokenADenom, tokenBDenom string) (
 ) {
 	Aasset, found1 := getPoolAssetByDenom(p.PoolAssets, tokenADenom)
 	Basset, found2 := getPoolAssetByDenom(p.PoolAssets, tokenBDenom)
-	if !(found1 && found2) {
-		return Aasset, Basset, errors.New("one of the provided pool denoms does not exist in pool")
+
+	if !found1 {
+		return PoolAsset{}, PoolAsset{}, fmt.Errorf("(%s) does not exist in the pool", tokenADenom)
+	}
+	if !found2 {
+		return PoolAsset{}, PoolAsset{}, fmt.Errorf("(%s) does not exist in the pool", tokenBDenom)
 	}
 	return Aasset, Basset, nil
 }
@@ -261,7 +265,10 @@ func (p Pool) parsePoolAssets(tokensA sdk.Coins, tokenBDenom string) (
 		return tokenA, Aasset, Basset, errors.New("expected tokensB to be of length one")
 	}
 	Aasset, Basset, err = p.parsePoolAssetsByDenoms(tokensA[0].Denom, tokenBDenom)
-	return tokensA[0], Aasset, Basset, err
+	if err != nil {
+		return sdk.Coin{}, PoolAsset{}, PoolAsset{}, err
+	}
+	return tokensA[0], Aasset, Basset, nil
 }
 
 func (p Pool) parsePoolAssetsCoins(tokensA sdk.Coins, tokensB sdk.Coins) (