/hubs/N/tags/nonexistent shouldn't redirect to /hubs/N/items

[zackw]

If you try to load the tags page for a nonexistent tag, it silently redirects you to the "all items" screen for the hub, instead of telling you that there are no items with that tag. This made me think #49 was some sort of catastrophic database bug instead of a mere rendering problem.

[zackw]

The tags controller tries to show an error message ...

    unless @tag
      flash.now[:error] = "We're sorry, but '#{params[:name]}' is not a tag for '#{@hub.title}'"

      redirect_to hub_path(@hub)
    end

... but the redirection drops the flash on the floor. It should be flash[:error] = ....