Notion: check for errors faster

[madebyisaacr]

Description

This pull request makes slug error checking faster in Notion by checking slugs before fetching page content. Syncing is cancelled and errors are reported instantly after all database items have been fetched instead of waiting to fetch all pages' content before checking for errors.

Also improved the progress bar loading percentage for databases with less than 20 items by instantly jumping to the full loading phase value.

Closes #565

Changelog

• Slug errors such as missing or duplicate slug values are now reported much faster when syncing large databases.

Testing

Database for testing: https://www.notion.so/framer/224adf6e8c9680eca17fda0e00267005?v=224adf6e8c9680899fb5000c4fa462a1

• Sync a database with duplicate slugs.
• Sync a database with a missing slug.

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