Ignore errors from transaction rollbacks

app/lib/backend.dart

Index: app/lib/backend.dart
diff --git a/app/lib/backend.dart b/app/lib/backend.dart
index 6234b7f0e594fc32a2e08e3081749b1ed77e9fe9..69f204ed5b35b4a7ef69e7914f6a34f87544259b 100644
--- a/app/lib/backend.dart
+++ b/app/lib/backend.dart
@@ -314,7 +314,15 @@ class GCloudPackageRepository extends PackageRepository {
             newVersion.pubspec.jsonString);
       } catch (error, stack) {
         _logger.warning('Error while committing: $error, $stack');
-        await T.rollback();
+
+        // This call might fail if the transaction has already been
+        // committed/rolled back or the transaction failed.
+        //
+        // In which case we simply ignore the rollback error and rethrow the
+        // original error.
+        try {
+          await T.rollback();
+        } catch (_) {}
         rethrow;
       }
     });