encoding/protobuf: don't use disallowed "__" prefix

Change-Id: If143743b952f993cb97c3493361610c7f10a5d2f
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3580
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/protobuf/protobuf_test.go b/encoding/protobuf/protobuf_test.go
index bd5a905..b824e05 100644
--- a/encoding/protobuf/protobuf_test.go
+++ b/encoding/protobuf/protobuf_test.go
@@ -25,6 +25,7 @@
 	"testing"
 
 	"cuelang.org/go/cue/ast"
+	"cuelang.org/go/cue/errors"
 	"cuelang.org/go/cue/format"
 	"github.com/kr/pretty"
 )
@@ -92,7 +93,7 @@
 
 	files, err := b.Files()
 	if err != nil {
-		t.Fatal(err)
+		t.Fatal(errors.Details(err, nil))
 	}
 
 	if *update {