Print out count of images in /aiinput on startup

[neilenns]

Fixes #330

Description of changes

• Stop startup if any of the watch folders don't exist
• Log errors appropriately
• If the watch folders do exist log the count of images in it

Checklist

If your change touches anything under src or the README.md file these items must be done:

• [X] User-facing change description added to unreleased section of CHANGELOG.md