message: return if initializing a rebase fails.
diff: @@ -889,8 +889,10 @@ void Repository::rebase(const AnnotatedCommit &mergeHead,
  git_rebase_init(&r, d->repo, nullptr, mergeHead, nullptr, &opts);
  auto rebase = git::Rebase(d->repo, r, overrideUser, overrideEmail);
  - if (!rebase.isValid())
  + if (!rebase.isValid()) {
  emit d->notifier->rebaseInitError();
  + return;
  + }
  // start rebasing
  rebaseContinue(QStringLiteral(""));
  