Enable raise_on_missing_only in development (#6495)

Configure Plug.Static with raise_on_missing_only: code_reloading?() to help
developers catch missing entries in static_paths() during development. This
prevents the common trap where files like favicon.svg return 404 because they
aren't properly declared in the :only list (often because they digested paths).

Closes #6203

Author: brendon9x

installer/templates/phx_web/endpoint.ex

@@ -24,7 +24,8 @@ defmodule <%= @endpoint_module %> do
     at: "/",
     from: :<%= @web_app_name %>,
     gzip: not code_reloading?,
-    only: <%= @web_namespace %>.static_paths()
+    only: <%= @web_namespace %>.static_paths(),
+    raise_on_missing_only: code_reloading?
 
   # Code reloading can be explicitly enabled under the
   # :code_reloader configuration of your endpoint.