Improve create handling of a frontmatter page_id

[willkg]

create currently, for a file that already has a page_id:

• if the page exists → errors "a page already exists at this page_id" (no link);
• if the id doesn't resolve → silently creates a new page and overwrites the id on write-back.

Improve both:

• page exists → include the URL of the existing page in the error;
• page_id present but bad/nonexistent → error that the id is bad and should be removed, rather than silently creating a new page.

(From the old todo.md.)