Handle ValueTranscoder EncodeTo returns error on bad type

Instead of panicking

example_value_transcoder_test.go

@@ -55,9 +55,15 @@ func decodePointFromText(mr *pgx.MessageReader, size int32) interface{} {
 	return p
 }
 
-func encodePoint(w *pgx.MessageWriter, value interface{}) {
-	p := value.(Point)
+func encodePoint(w *pgx.MessageWriter, value interface{}) error {
+	p, ok := value.(Point)
+	if !ok {
+		return fmt.Errorf("Expected Point, received %T", value)
+	}
+
 	s := fmt.Sprintf("point(%v,%v)", p.x, p.y)
 	w.Write(int32(len(s)))
 	w.WriteString(s)
+
+	return nil
 }