UI - Add extra validation help for notification body with Jinja2 markup (#1810)

changedetectionio/forms.py

@@ -229,16 +229,19 @@ class ValidateJinja2Template(object):
     def __call__(self, form, field):
         from changedetectionio import notification
 
-        from jinja2 import Environment, BaseLoader, TemplateSyntaxError
+        from jinja2 import Environment, BaseLoader, TemplateSyntaxError, UndefinedError
         from jinja2.meta import find_undeclared_variables
 
 
         try:
             jinja2_env = Environment(loader=BaseLoader)
             jinja2_env.globals.update(notification.valid_tokens)
+
             rendered = jinja2_env.from_string(field.data).render()
         except TemplateSyntaxError as e:
             raise ValidationError(f"This is not a valid Jinja2 template: {e}") from e
+        except UndefinedError as e:
+            raise ValidationError(f"A variable or function is not defined: {e}") from e
 
         ast = jinja2_env.parse(field.data)
         undefined = ", ".join(find_undeclared_variables(ast))