Improve error message for nil values in repeated fields.

Also unexport ErrRepeatedHasNil, since there's no utility in checking for it.
diff --git a/proto/all_test.go b/proto/all_test.go
index 3fade17..aa67b10 100644
--- a/proto/all_test.go
+++ b/proto/all_test.go
@@ -1252,7 +1252,8 @@
 	}
 
 	o := old()
-	if err := o.Marshal(pb); err != ErrRepeatedHasNil {
+	err := o.Marshal(pb)
+	if err == nil || !strings.Contains(err.Error(), "repeated field Message has nil") {
 		t.Fatalf("unexpected or no error when marshaling: %v", err)
 	}
 }