kubernetes_node_taint: don't fail if node is missing (#2099)

Show a warning instead of an error if a tainted node is missing. 
Otherwise, taints for deleted nodes can only be removed via `terraform state rm`.

.changelog/2099.txt

@@ -0,0 +1,4 @@
+```release-note:bug
+`resource/kubernetes_node_taint`: Don't fail when there is a taint in the state file for a node that no longer exists.
+
+```

kubernetes/resource_kubernetes_node_taint.go

@@ -100,6 +100,14 @@ func resourceKubernetesNodeTaintRead(ctx context.Context, d *schema.ResourceData
 
 	conn, err := m.(KubeClientsets).MainClientset()
 	if err != nil {
+		if statusErr, ok := err.(*errors.StatusError); ok && errors.IsNotFound(statusErr) {
+			// The node is gone so the resource should be deleted.
+			return diag.Diagnostics{{
+				Severity: diag.Warning,
+				Summary:  "Node has been deleted",
+				Detail:   fmt.Sprintf("The underlying node %q has been deleted. You should remove it from your configuration.", nodeName),
+			}}
+		}
 		return diag.FromErr(err)
 	}
 	nodeApi := conn.CoreV1().Nodes()