Update the error htmls to check if the flash exists.

lib/decentralised_book_index_web/controllers/error_html/404.html.heex

@@ -18,7 +18,9 @@
 
     <main class="px-4 pt-20 pb-2 sm:px-6 lg:px-8">
       <div class="mx-auto max-w-4xl">
+        <%= if Map.has_key?(assigns, :flash) do %>
           <.flash_group flash={@flash} />
+        <% end %>
 
         <section class="bg-white dark:bg-gray-900">
           <div class="py-8 px-4 mx-auto max-w-screen-xl lg:py-16 lg:px-6">

lib/decentralised_book_index_web/controllers/error_html/500.html.heex

@@ -18,7 +18,9 @@
 
     <main class="px-4 pt-20 pb-2 sm:px-6 lg:px-8">
       <div class="mx-auto max-w-4xl">
+        <%= if Map.has_key?(assigns, :flash) do %>
           <.flash_group flash={@flash} />
+        <% end %>
 
         <section class="bg-white dark:bg-gray-900">
           <div class="py-8 px-4 mx-auto max-w-screen-xl lg:py-16 lg:px-6">