fix iptables IsNotExist error comparison

The comparison done in the IsNotExist method is missing
the parenthesis aggregating the logical or statements,
thus the expression evaluation is not working as
expected.

Signed-off-by: Antonio Ojea <antonio.ojea.garcia@gmail.com>

iptables/iptables.go

@@ -48,9 +48,13 @@ func (e *Error) Error() string {
 
 // IsNotExist returns true if the error is due to the chain or rule not existing
 func (e *Error) IsNotExist() bool {
-	return e.ExitStatus() == 1 &&
-		strings.Contains(e.msg, fmt.Sprintf("%s: Bad rule (does a matching rule exist in that chain?).\n", getIptablesCommand(e.proto))) ||
-		strings.Contains(e.msg, fmt.Sprintf("%s: No chain/target/match by that name.\n", getIptablesCommand(e.proto)))
+	if e.ExitStatus() != 1 {
+		return false
+	}
+	cmdIptables := getIptablesCommand(e.proto)
+	msgNoRuleExist := fmt.Sprintf("%s: Bad rule (does a matching rule exist in that chain?).\n", cmdIptables)
+	msgNoChainExist := fmt.Sprintf("%s: No chain/target/match by that name.\n", cmdIptables)
+	return strings.Contains(e.msg, msgNoRuleExist) || strings.Contains(e.msg, msgNoChainExist)
 }
 
 // Protocol to differentiate between IPv4 and IPv6