[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/404.html.heex

@@ -0,0 +1,3 @@
+<section>
+  <h2 class="text-title-md2 font-bold text-white">404 (not found)</h2>
+</section>