forms: fix form validation error messages.

* Fix it so that all edit forms show an error banner if the form
  has validation errors. Previously forms had to manually call
  `error_messages_for`, which not all forms did.

* Fix it so that the full validation error message is shown next to each
  input attribute that had errors. Also update the styling of these
  error messages to look better.

app/helpers/application_helper.rb

@@ -92,16 +92,6 @@ module ApplicationHelper
     DText.strip_dtext(text)
   end
 
-  def error_messages_for(instance_name)
-    instance = instance_variable_get("@#{instance_name}")
-
-    if instance&.errors&.any?
-      %{<div class="error-messages ui-state-error ui-corner-all"><strong>Error</strong>: #{instance.__send__(:errors).full_messages.join(", ")}</div>}.html_safe
-    else
-      ""
-    end
-  end
-
   def time_tag(content, time, **options)
     datetime = time.strftime("%Y-%m-%dT%H:%M%:z")
 
@@ -256,7 +246,14 @@ module ApplicationHelper
   def edit_form_for(model, **options, &block)
     options[:html] = { autocomplete: "off", **options[:html].to_h }
     options[:authenticity_token] = true if options[:remote] == true
-    simple_form_for(model, **options, &block)
+
+    simple_form_for(model, **options) do |form|
+      if model.try(:errors).try(:any?)
+        concat tag.div(model.errors.full_messages.join("; "), class: "notice notice-error notice-small")
+      end
+
+      block.call(form)
+    end
   end
 
   def table_for(...)