Addons: return 404 when the project does not exist in the DB

[humitos]

Found via Sentry at
https://read-the-docs.sentry.io/issues/5206873079/events/66c67d38c3a74ac1a4048b8aad3f1f55/

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: readthedocs/proxito/views/hosting.py

Function	Unhandled Issue
get	AttributeError: 'NoneType' object has no attribute 'supports_multiple_versions' ... Event Count: 4.3k

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