doc: add halted to demo

If a event chain is halted, it fires the 'V:eventsHalted' event.
That can be handled so that the default error handling does not kick in and you can handle it yourself.

The post was issuing a multi-part post with body. That is a malformed request and starting with Rack 2.2.2 will generate an exception. rack/rack#1603
This fixes that issue.

app/demo/events/halted.pom

@@ -0,0 +1,23 @@
+Voom::Presenters.define(:halted) do
+  helpers Demo::Helpers::IndentedGrid
+  attach :top_nav
+  attach :events_drawer
+
+  indented_grid do
+    display 'Halted Events'
+    body 'This demonstrates how to respond to failed, or halted events'
+
+    content do
+      button :failed_post do
+        event :click do
+          posts '_echo_', status: 500
+        end
+        event 'V:eventsHalted' do
+          snackbar 'That did not work! Please try again later.'
+        end
+      end
+
+    end
+  end
+  attach :code, file: __FILE__
+end

app/demo/events/nav/drawer.pom

@@ -1,7 +1,7 @@
 Voom::Presenters.define(:events_drawer) do
   helpers Voom::Presenters::Helpers::Inflector
 
-  events = %i[field_level_events form_level_events tagged_input parallel_events].sort
+  events = %i[field_level_events form_level_events tagged_input parallel_events halted].sort
   actions = %i[dialog replaces loads toggle_visibility snackbar updates deletes posts clear last_response prompt_if_dirty].sort
 
   drawer 'Events' do