Handle usergroup already exists error (#71)

Thank you so much, the provider has nice handling when a deactivated usergroup exists. But looks like slack changed the error message in the API. The error thrown is now called `handle_already_exists`:

```

│ Error: could not create usergroup <xxxxx>: handle_already_exists
│ 
│   with slack_usergroup.<xxxxx>,
│   on gen_slack.tf line 1867, in resource "slack_usergroup" "<xxxxx>":
│ 1867:   resource "slack_usergroup" "<xxxxx>" {
```

slack/resource_usergroup.go

@@ -73,7 +73,7 @@ func resourceSlackUserGroupCreate(ctx context.Context, d *schema.ResourceData, m
 	}
 	createdUserGroup, err := client.CreateUserGroupContext(ctx, userGroup)
 	if err != nil {
-		if err.Error() != "name_already_exists" {
+		if err.Error() != "name_already_exists" && err.Error() != "handle_already_exists" {
 			return diag.Errorf("could not create usergroup %s: %s", name, err)
 		}
 		group, err := findUserGroupByName(ctx, name, true, m)