Better error handling for CI dependency errors
Time and again I've seen people struggle with writing correct .kde-ci.yml files. I think we can improve the error handling to help people figure out the issues on their own.
There are two main classes of issues:
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Wrong project names. This can be typos, or people using project paths (kde/applications/thing) instead of repo paths (graphics/thing). In both cases the dependency is just silently ignored. We probably should raise an error if the dependency is not found (with the exception of third-party/*).
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Missing branch rules. In this case there is an error message, which sounds like the dependency does not exist. Instead the error should point out that the project does exist, but a branch rule is missing and suggest how to add it