[lectures/02-interfaces] Add errors.As example.

lectures/02-interfaces/lecture.slide

@@ -475,3 +475,7 @@ As
   if perr, ok := err.(*os.PathError); ok {
       fmt.Println(perr.Path)
   }
+
+* errors.As
+
+.play temporary/main.go

lectures/02-interfaces/temporary/main.go

@@ -0,0 +1,18 @@
+package main
+
+import "errors"
+
+type Temporary interface {
+	IsTemporary() bool
+}
+
+func do() error { return nil }
+
+func main() {
+	err := do()
+
+	var terr Temporary
+	if errors.As(err, &terr) && terr.IsTemporary() {
+		//...
+	}
+}