Address more review feedback

Use `errors.Is` instead of `errors.As` since we don't need to access special
fields in the custom error type.

Rather than generating a new error to wrap, just capture the exisitng error.

eth/catalyst/simulated_beacon.go

@@ -88,7 +88,7 @@ type errTxPoolTerminated struct {
 
 // Error returns the message from the wrapped error.
 func (t *errTxPoolTerminated) Error() string {
-	return t.error.Error()
+	return fmt.Errorf("failed to sync txpool: %w", t.error).Error()
 }
 
 // SimulatedBeacon drives an Ethereum instance as if it were a real beacon
@@ -192,7 +192,7 @@ func (c *SimulatedBeacon) sealBlock(withdrawals []*types.Withdrawal, timestamp u
 	// behavior, the pool will be explicitly blocked on its reset before
 	// continuing to the block production below.
 	if err := c.eth.APIBackend.TxPool().Sync(); err != nil {
-		return &errTxPoolTerminated{fmt.Errorf("failed to sync txpool: %w", err)}
+		return &errTxPoolTerminated{err}
 	}
 
 	version := payloadVersion(c.eth.BlockChain().Config(), timestamp)

eth/catalyst/simulated_beacon_api.go

@@ -73,8 +73,7 @@ func (a *simulatedBeaconAPI) loop() {
 				}
 				// Avoids spinlooping if the txPool is alredy terminated.
 				if _, err := a.sim.commit(); err != nil {
-					var txpTermErr *errTxPoolTerminated
-					if errors.As(err, &txpTermErr) {
+					if errors.Is(err, &errTxPoolTerminated{}) {
 						break
 					}
 				}