feat(tm2): improve error message on invalid coins

[MikaelVallenet]

fix #4930

I know TM2 is a "foundational" component we avoid to change.
I met a problem with not explicit error i tried to fix it, i could fix it in the IssueCoin fn here but felt like it make more sense in tm2.

// IssueCoin implements the Banker interface.
func (tb *TestBanker) IssueCoin(addr crypto.Bech32Address, denom string, amt int64) {
	coins := tb.CoinTable[addr]
	sum := coins.Add(tm2std.Coins{{Denom: denom, Amount: amt}})
	tb.CoinTable[addr] = sum
}

I was also thinking of improving this error message, but to keep it scoped i did not do it in this PR

func (e InvalidCoinsError) Error() string       { return "invalid coins error" }

This error is used why coins are not valid, but it does not explain what is a valid coin (sorted, positive amount, lowercase denom)

[Gno2D2]

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Codecov Report

❌ Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
tm2/pkg/std/coin.go	77.77%	4 Missing ⚠️

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[jefft0]

@MikaelVallenet, are you able to fix the CI checks?

[MikaelVallenet]

@MikaelVallenet, are you able to fix the CI checks?

done ✅