Change categories when editing a rule

#165 
this works:
```php
if (strpos($e->getMessage(), 'INVALID_ITEM') !== false)
{
	throw new \phpbb\boardrules\exception\out_of_bounds('rule_id');
}
else
{
	throw new \phpbb\boardrules\exception\out_of_bounds('new_parent_id');
}
```