Admin project controller should notify on transfer error

Summary

The Admin project controller doesn't present errors from the underlying transfer service if one is encountered.

Steps to reproduce

Create a personal repository with tagged docker images in the registry. Ensure there's a group available that you are not an "Owner" of. Via the admin project path, attempt to move the project to that available group See that it doesn't transfer

What is the current bug behavior?

The admin project controller does not flash a message when an error occurs within the transfer service

What is the expected correct behavior?

There should be some indication that there was an error during this process

Possible fixes

https://gitlab.com/gitlab-org/gitlab-foss/-/blob/master/app/controllers/admin/projects_controller.rb#L56

  # rubocop: disable CodeReuse/ActiveRecord
  def transfer
    namespace = Namespace.find_by(id: params[:new_namespace_id])
    ::Projects::TransferService.new(@project, current_user, params.dup).execute(namespace)

    @project.reset
    redirect_to admin_project_path(@project)
  end

The transfer service does raise an error and then stashes it in project.errors

    def execute(new_namespace)
...
      transfer(project)

      current_user.invalidate_personal_projects_count

      true
    rescue Projects::TransferService::TransferError => ex
      project.reset
      project.errors.add(:new_namespace, ex.message)
      false
    end

    # rubocop: disable CodeReuse/ActiveRecord
    def transfer(project)
      @old_path = project.full_path
      @old_group = project.group
      @new_path = File.join(@new_namespace.try(:full_path) || '', project.path)
      @old_namespace = project.namespace

      if Project.where(namespace_id: @new_namespace.try(:id)).where('path = ? or name = ?', project.path, project.name).exists?
        raise TransferError.new(s_("TransferProject|Project with same name or path in target namespace already exists"))
      end

      if project.has_container_registry_tags?
        # We currently don't support renaming repository if it contains tags in container registry
        raise TransferError.new(s_('TransferProject|Project cannot be transferred, because tags are present in its container registry'))
      end

      attempt_transfer_transaction
    end

Something akin to the below may work (I have not tested it)

  # rubocop: disable CodeReuse/ActiveRecord
  def transfer
    namespace = Namespace.find_by(id: params[:new_namespace_id])
    ::Projects::TransferService.new(@project, current_user, params.dup).execute(namespace)

    if @project.errors[:new_namespace].present?
      flash[:alert] = @project.errors[:new_namespace].first
    end

    @project.reset
    redirect_to admin_project_path(@project)
  end