Check if file exists before adding. Fix #69

[moonso]

This PR adds a check if a file exists when adding from CLI

How to test:

1. Tests are automated

Expected outcome: housekeeper should abort if trying to add a file that does not exist.

Review:

• [x] code approved by HS
• [x] tests executed by GitHub Actions
• [x] "Merge and deploy" approved by @moonso Thanks for filling in who performed the code review and the test!

This is patch version bump

[moonso]

Merged and deployed on hasta