resource/aws_network_acl: Fix issue with updating subnet associations…

… returning InvalidAssociationID.NotFound (#13382)

Output from acceptance testing:

```
--- PASS: TestAccAWSNetworkAcl_basic (37.40s)
--- PASS: TestAccAWSNetworkAcl_CaseSensitivityNoChanges (51.54s)
--- PASS: TestAccAWSNetworkAcl_disappears (22.76s)
--- PASS: TestAccAWSNetworkAcl_Egress_ConfigMode (75.61s)
--- PASS: TestAccAWSNetworkAcl_EgressAndIngressRules (60.15s)
--- PASS: TestAccAWSNetworkAcl_espProtocol (18.73s)
--- PASS: TestAccAWSNetworkAcl_Ingress_ConfigMode (97.83s)
--- PASS: TestAccAWSNetworkAcl_ipv6ICMPRules (36.94s)
--- PASS: TestAccAWSNetworkAcl_ipv6Rules (45.59s)
--- PASS: TestAccAWSNetworkAcl_ipv6VpcRules (27.91s)
--- PASS: TestAccAWSNetworkAcl_OnlyEgressRules (56.36s)
--- PASS: TestAccAWSNetworkAcl_OnlyIngressRules_basic (52.51s)
--- PASS: TestAccAWSNetworkAcl_OnlyIngressRules_update (53.47s)
--- PASS: TestAccAWSNetworkAcl_SubnetChange (67.85s)
--- PASS: TestAccAWSNetworkAcl_Subnets (50.49s)
--- PASS: TestAccAWSNetworkAcl_SubnetsDelete (40.37s)
```

aws/resource_aws_network_acl.go

@@ -303,13 +303,16 @@ func resourceAwsNetworkAclUpdate(d *schema.ResourceData, meta interface{}) error
 					}
 					return fmt.Errorf("Failed to find acl association: acl %s with subnet %s: %s", d.Id(), r, err)
 				}
-				log.Printf("DEBUG] Replacing Network Acl Association (%s) with Default Network ACL ID (%s)", *association.NetworkAclAssociationId, *defaultAcl.NetworkAclId)
+				log.Printf("[DEBUG] Replacing Network Acl Association (%s) with Default Network ACL ID (%s)", *association.NetworkAclAssociationId, *defaultAcl.NetworkAclId)
 				_, err = conn.ReplaceNetworkAclAssociation(&ec2.ReplaceNetworkAclAssociationInput{
 					AssociationId: association.NetworkAclAssociationId,
 					NetworkAclId:  defaultAcl.NetworkAclId,
 				})
 				if err != nil {
-					return err
+					if isAWSErr(err, "InvalidAssociationID.NotFound", "") {
+						continue
+					}
+					return fmt.Errorf("Error Replacing Default Network Acl Association: %s", err)
 				}
 			}
 		}