Make `ProjectRead` not return `Option` for info and meta

[Jonas-Puksta-Sensmetry]

Currently ProjectRead::get_project returns

Result<(Option<InterchangeProjectInfoRaw>, Option<InterchangeProjectMetadataRaw>), Self::Error>

So everyone who calls this needs to report that ".project.json is missing" or ".meta.json is missing", and add the path/url to the error, which is often not done.

It would be much more convenient for everyone if each ProjectRead implementation would have an extra error case for missing info/meta. It would also be easier to include the relevant context (such as path/url) in the error there.