refactor (*UUID).UnmarshalText (#58)

Now it's easier to set breakpoint on error for debugging.
diff --git a/marshal.go b/marshal.go
index 7f9e0c6..14bd340 100644
--- a/marshal.go
+++ b/marshal.go
@@ -16,10 +16,11 @@
 // UnmarshalText implements encoding.TextUnmarshaler.
 func (uuid *UUID) UnmarshalText(data []byte) error {
 	id, err := ParseBytes(data)
-	if err == nil {
-		*uuid = id
+	if err != nil {
+		return err
 	}
-	return err
+	*uuid = id
+	return nil
 }
 
 // MarshalBinary implements encoding.BinaryMarshaler.