Show errors on new task order form when fields are invalid

Form errors were not being shown because a new form instance was being
created each time the `form` property was being called. Since the
`validate` method on the form adds the errors to the form instance, this
was causing no errors to be shown even if the form were invalid.

atst/routes/task_orders/new.py

@@ -108,10 +108,11 @@ class UpdateTaskOrderWorkflow(ShowTaskOrderWorkflow):
         self.task_order_id = task_order_id
         self._task_order = None
         self._section = TASK_ORDER_SECTIONS[screen - 1]
+        self._form = self._section["form"](self.form_data)
 
     @property
     def form(self):
-        return self._section["form"](self.form_data)
+        return self._form
 
     @property
     def workspace(self):