Update the tests for the error htmls.

test/decentralised_book_index_web/controllers/error_html_test.exs

@@ -5,11 +5,18 @@ defmodule DecentralisedBookIndexWeb.ErrorHTMLTest do
   import Phoenix.Template
 
   test "renders 404.html" do
-    assert render_to_string(DecentralisedBookIndexWeb.ErrorHTML, "404", "html", []) == "Not Found"
+    html = render_to_string(DecentralisedBookIndexWeb.ErrorHTML, "404", "html", [])
+    assert html =~ "404"
+    assert html =~ "Something's missing."
+    assert html =~ "Sorry, we can't find that page."
+    assert html =~ "Go to books"
   end
 
   test "renders 500.html" do
-    assert render_to_string(DecentralisedBookIndexWeb.ErrorHTML, "500", "html", []) ==
-             "Internal Server Error"
+    html = render_to_string(DecentralisedBookIndexWeb.ErrorHTML, "500", "html", [])
+    assert html =~ "500"
+    assert html =~ "Something's went wrong."
+    assert html =~ "Try to refresh the page. Feel free to contact us if the problem persists."
+    assert html =~ "Go to books"
   end
 end