Derive status from internal transaction at index 0

Why:

* Issue #757 was created after we noticed a few constraint violations
for the `error` column in the `transactions` table. The constraint
violations came about as we attempted to update transactions with a
successful status but some value for the error field. The constraint
is violated in this scenario because we expect successful transactions
to not have an error. [EIP 658](https://github.com/ethereum/EIPs/pull/658),
which apparently is also called [EIP 98](https://github.com/ethereum/EIPs/issues/98#issuecomment-318670322)
, says that the status should be set to 1 (success) if the outermost
code execution succeeded (internal transaction with index 0), or it
should be set to 0 (failure) if the outermost code execution failed.
We're currently deriving the status from the last internal transaction
when in fact we should be deriving it from the first internal
transaction.
* Issue link: https://github.com/poanetwork/blockscout/issues/757

This change addresses the need by:

* Editing `Explorer.Chain.Import.update_transactions/2` to set the error
for a transaction equal to the error, if any, of the internal
transaction with index 0.
* Editing `Explorer.Chain.Import.update_transactions/2` to set a
transaction's status as successful if it's internal transaction at index
0 doesn't have an error, and as failed if it does. This only applies to
pre-Byzantium and Ethereum Classic, for which transaction status is not
set from receipts.

apps/explorer/lib/explorer/chain.ex

@@ -1384,12 +1384,12 @@ defmodule Explorer.Chain do
     * `:pending` - the transaction has not be confirmed in a block yet.
     * `:awaiting_internal_transactions` - the transaction happened in a pre-Byzantium block or on a chain like Ethereum
       Classic (ETC) that never adopted [EIP-658](https://github.com/Arachnid/EIPs/blob/master/EIPS/eip-658.md), which
-      add transaction status to transaction receipts, so the stats can only be derived whether the last internal
+      add transaction status to transaction receipts, so the status can only be derived whether the first internal
       transaction has an error.
     * `:success` - the transaction has been confirmed in a block
     * `{:error, :awaiting_internal_transactions}` - the transactions happened post-Byzantium, but the error message
        requires the internal transactions.
-    * `{:error, reason}` - the transaction failed due to `reason` in its last internal transaction.
+    * `{:error, reason}` - the transaction failed due to `reason` in its first internal transaction.
 
   """
   @spec transaction_to_status(Transaction.t()) ::

apps/explorer/lib/explorer/chain/import.ex

@@ -868,12 +868,12 @@ defmodule Explorer.Chain.Import do
               ),
             error:
               fragment(
-                "(SELECT it.error FROM internal_transactions AS it WHERE it.transaction_hash = ? ORDER BY it.index DESC LIMIT 1)",
+                "(SELECT it.error FROM internal_transactions AS it WHERE it.transaction_hash = ? ORDER BY it.index ASC LIMIT 1)",
                 t.hash
               ),
             status:
               fragment(
-                "COALESCE(?, CASE WHEN (SELECT it.error FROM internal_transactions AS it WHERE it.transaction_hash = ? ORDER BY it.index DESC LIMIT 1) IS NULL THEN ? ELSE ? END)",
+                "COALESCE(?, CASE WHEN (SELECT it.error FROM internal_transactions AS it WHERE it.transaction_hash = ? ORDER BY it.index ASC LIMIT 1) IS NULL THEN ? ELSE ? END)",
                 t.status,
                 t.hash,
                 type(^:ok, t.status),