addToWatchlist raise NotFound exception for invalid media

[JonnyWong16]

Description

Plex now raises 404 status when adding invalid media to a Watchlist. Documentation updated to say this will raise a NotFound exception. Test updated to check for the NotFound exception.

https://github.com/pkkid/python-plexapi/actions/runs/8756988631/job/24034937906#step:12:837

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable