debian_pipeline should error out when fed with a .changes with binary packages on top of the source package

In #954 (closed), we fixed MergeChanges to cope with that case and error out instead of doing something entirely wrong. It would still be cleaner to error out much earlier in the workflow, in particular since you can have workflows that doesn't rely on MergeChanges and that might not error out and do something entirely wrong when they get binaries on top of the expected source packages.

(Or we can go in the opposite direction, accept the binaries and record them so that we don't try to rebuild them, etc. but that seems going in the wrong direction)