[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

lib/pinchflat_web/controllers/error_html.ex

@@ -1,14 +1,7 @@
 defmodule PinchflatWeb.ErrorHTML do
   use PinchflatWeb, :html
 
-  # If you want to customize your error pages,
-  # uncomment the embed_templates/1 call below
-  # and add pages to the error directory:
-  #
-  #   * lib/pinchflat_web/controllers/error_html/404.html.heex
-  #   * lib/pinchflat_web/controllers/error_html/500.html.heex
-  #
-  # embed_templates "error_html/*"
+  embed_templates "error_html/*"
 
   # The default is to render a plain text page based on
   # the template name. For example, "404.html" becomes