Fix default case handling in walkChild

A refreshed understanding has been etched in commentary.

example_test.go

@@ -2,7 +2,6 @@ package stick_test
 
 import (
 	"bytes"
-	"errors"
 	"fmt"
 	"io/ioutil"
 	"os"

exec.go

@@ -344,7 +344,8 @@ func (s *state) walkChild(node parse.Node) error {
 	case *parse.UseNode:
 		return s.walkUseNode(node)
 	default:
-		return fmt.Errorf("walkChild: unsupported Node type: %T (bug?)", node)
+		// No need to handle other nodes. This function only populates blocks from a
+		// referenced template (in a use statement) and does not actually execute anything.
 	}
 	return nil
 }