Handle instances not found in attached disk resource (hashicorp#497)

<!-- This change is generated by MagicModules. -->
/cc @rileykarson

google-beta/resource_compute_attached_disk.go

@@ -126,7 +126,7 @@ func resourceAttachedDiskRead(d *schema.ResourceData, meta interface{}) error {
 
 	instance, err := config.clientCompute.Instances.Get(zv.Project, zv.Zone, zv.Name).Do()
 	if err != nil {
-		return err
+		return handleNotFoundError(err, d, fmt.Sprintf("AttachedDisk %q", d.Id()))
 	}
 
 	// Iterate through the instance's attached disks as this is the only way to