Improve error message with invalid keys

`page.type_arrow_down()` and `page.type_down_arrow()` should either both work or provide a better message.

Start by comparing the page.type_arrow_down() and page.type_down_arrow() entry points named in the issue. Determine whether both key names should work or whether the invalid-key error should be improved; done means the two calls behave consistently or the resulting message clearly explains the valid choice.

Written by the indexing model from the issue text.