add custom exceptions handling controller

app/controllers/errors_controller.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+class ErrorsController < ActionController::Base
+  ENTERPRISE_FEATURE_MESSAGE =
+    'This feature is available in Enterprise Edition: https://www.docuseal.co/pricing'
+
+  ENTERPRISE_PATHS = [
+    '/templates/html',
+    '/api/templates/html',
+    '/templates/pdf',
+    '/api/templates/pdf'
+  ].freeze
+
+  def show
+    if request.original_fullpath.in?(ENTERPRISE_PATHS)
+      return render json: { status: 404, message: ENTERPRISE_FEATURE_MESSAGE }, status: :not_found
+    end
+
+    respond_to do |f|
+      f.json do
+        render json: { status: error_status_code }, status: error_status_code
+      end
+
+      f.html { render error_status_code.to_s, status: error_status_code }
+    end
+  end
+
+  private
+
+  def error_status_code
+    @error_status_code ||=
+      ActionDispatch::ExceptionWrapper.new(request.env,
+                                           request.env['action_dispatch.exception']).status_code
+  end
+end