Improve unlisted error when there is no files/ directory

lib/livebook_web/live/session_live/add_file_entry_unlisted_component.ex

@@ -16,7 +16,7 @@ defmodule LivebookWeb.SessionLive.AddFileEntryUnlistedComponent do
       |> assign_new(:files, fn ->
         case FileSystem.File.list(assigns.session.files_dir) do
           {:ok, files} -> unlisted_files(files, assigns.file_entries)
-          {:error, _} -> []
+          {:error, _} -> :none
         end
       end)
 
@@ -37,12 +37,16 @@ defmodule LivebookWeb.SessionLive.AddFileEntryUnlistedComponent do
   @impl true
   def render(assigns) do
     ~H"""
-    <div class="flex flex-col">
-      <%= if @files == [] do %>
-        <p class="text-gray-700">
-          No other files found in the <code>files/</code> directory.
-        </p>
-      <% else %>
+    <div>
+      <p :if={@files == :none} class="text-gray-700">
+        There is no <code>files/</code> directory that exists alongside your notebook.
+      </p>
+
+      <p :if={@files == []} class="text-gray-700">
+        No other files found in the <code>files/</code> directory.
+      </p>
+
+      <div :if={match?([_ | _], @files)} class="flex flex-col">
         <p class="text-gray-700">
           Here are other files from the <code>files/</code> directory that you may want to add.
         </p>
@@ -77,7 +81,7 @@ defmodule LivebookWeb.SessionLive.AddFileEntryUnlistedComponent do
             </.link>
           </div>
         </form>
-      <% end %>
+      </div>
     </div>
     """
   end