Error message when creating prototype page is uninformative

When you create a prototype page instance which uses a sitemap item name that is already in use by the main configuration, you get a "failed to create page" message without much explanation.

Reproduction:

create new prototype page instance

select existing url name (such as news if that feature is installed)

observed:

error message "Failed to create a new page"

expected/desired:

error message "Failed to create page: URL is already in use"