syncbase: more go vet fixes (that do not appear on darwin)

Change-Id: If3cd6efe86b5c924d3a02864aed37ac87b3c4236
diff --git a/services/syncbase/server/watchable/transaction.go b/services/syncbase/server/watchable/transaction.go
index f557ca7..304c4d6 100644
--- a/services/syncbase/server/watchable/transaction.go
+++ b/services/syncbase/server/watchable/transaction.go
@@ -75,7 +75,7 @@
 	tx.mu.Lock()
 	defer tx.mu.Unlock()
 	if tx.err != nil {
-		return &store.InvalidStream{tx.err}
+		return &store.InvalidStream{Error: tx.err}
 	}
 	var it store.Stream
 	if !tx.st.managesRange(start, limit) {