[Enhancement] Show error messages in-app for easier triage (#365)

* Added an improved error message screen with actionable details

* Added a basic 404 page

* fixed tests

test/pinchflat_web/controllers/error_html_test.exs

@@ -5,10 +5,10 @@ defmodule PinchflatWeb.ErrorHTMLTest do
   import Phoenix.Template
 
   test "renders 404.html" do
-    assert render_to_string(PinchflatWeb.ErrorHTML, "404", "html", []) == "Not Found"
+    assert render_to_string(PinchflatWeb.ErrorHTML, "404", "html", []) =~ "404 (not found)"
   end
 
   test "renders 500.html" do
-    assert render_to_string(PinchflatWeb.ErrorHTML, "500", "html", []) == "Internal Server Error"
+    assert render_to_string(PinchflatWeb.ErrorHTML, "500", "html", []) =~ "Internal Server Error"
   end
 end