internal/godoc: add ast.LabeledStmt

The ast.LabeledStmt type is registered for gob encoding. This is used by
source files for some packages, like blackfriday and goldmark.

Change-Id: I0666bf53eab9efd9098aa079d3cfdcf320a43fe5
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/258998
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/internal/godoc/encode.go b/internal/godoc/encode.go
index 7894003..4018480 100644
--- a/internal/godoc/encode.go
+++ b/internal/godoc/encode.go
@@ -51,6 +51,7 @@
 		&ast.IncDecStmt{},
 		&ast.IndexExpr{},
 		&ast.InterfaceType{},
+		&ast.LabeledStmt{},
 		&ast.MapType{},
 		&ast.ParenExpr{},
 		&ast.RangeStmt{},
@@ -92,10 +93,10 @@
 	enc := gob.NewEncoder(&buf)
 	// Encode the fset using the Write method it provides.
 	if err := p.Fset.Write(enc.Encode); err != nil {
-		return nil, err
+		return nil, fmt.Errorf("p.Fset.Write: %v", err)
 	}
 	if err := enc.Encode(p.gobPackage); err != nil {
-		return nil, err
+		return nil, fmt.Errorf("enc.Encode: %v", err)
 	}
 	for _, f := range p.Files {
 		fixupObjects(f.AST)