go/ast: fix formatting of error message

There are three problems:
1. There is no CR at the end of the message.
2. The message is unconditionally printed.
3. The message is printed to stdout.

Change-Id: Ib2d880eea03348e8a69720aad7752302a75bd277
Reviewed-on: https://go-review.googlesource.com/9622Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/go/ast/walk.go

@@ -361,8 +361,7 @@ func Walk(v Visitor, node Node) {
 		}
 
 	default:
-		fmt.Printf("ast.Walk: unexpected node type %T", n)
-		panic("ast.Walk")
+		panic(fmt.Sprintf("ast.Walk: unexpected node type %T", n))
 	}
 
 	v.Visit(nil)