Display and log a better error in case escrow transaction does not pass MinDelegationAmount

[ptrus]

Currently if an escrow transaction tries to delegate less than MinDelegationAmount the transaction fails with staking.ErrInvalidArgument and nothing is logged.

oasis-core/go/consensus/tendermint/apps/staking/transactions.go

Lines 178 to 181 in 6c6e703

	// Check if sender provided at least a minimum amount of stake.
	if escrow.Amount.Cmp(&params.MinDelegationAmount) < 0 {
	return staking.ErrInvalidArgument
	}

[kostko]

We could introduce a separate error for this, although this would be a consensus-breaking change.

[ptrus]

If we want to avoid a consensus-breaking change, then a good short-term improvement is just adding a log message.