Output more specific error message for "invalid" dependencies

[s-ludwig]

Currently DependencyResolver cannot distinguish between dependencies that don't exist and dependencies that have no available versions/branches (unusual, but theoretically possible). A flag needs to be added to make that distinction and to enable outputting a clear error message ("Package x references non-existent package y.").

[WebFreak001]

error output has improved since 2015, new issues should be more specific what kind of improvements are needed.