Fix transaction failed to buy gas

[annakaz]

Created issue from #809 (comment)

So this is an error because if a transaction is being processed here, it is assumed that it was already validated by the transaction pool, in which case this should never happen. We are breaking that assumption in the EstimateGas function. I think the best way to handle this would be to run the transaction through transaction pool validation, either in its entirety or just to validate that the transaction can pay its fees, before applying the state transition.
Here is the validateTx function on TxPool, which you obviously can't call directly as it stands

celo-blockchain/core/tx_pool.go

Line 607 in f86d673

func (pool *TxPool) validateTx(tx *types.Transaction, local bool) error {

And here is the part that validates balances to ensure fees can be paid

celo-blockchain/core/tx_pool.go

Line 1201 in f86d673

func ValidateTransactorBalanceCoversTx(tx *types.Transaction, from common.Address, currentState *state.StateDB) error {

Two courses of action would either be to export the former, or just call the latter if that turns out to be inappropriate. The call should probably occur here-ish

celo-blockchain/internal/ethapi/api.go

Line 740 in f86d673

If this seems onerous, please leave this log statement as is and copy the above into an issue to address later. And feel free to assign me to it.

[gastonponti]

Issue already fixed with the PR #825