Check missing artifacts after a dependency graph result is received

[pkocandr]

Indy/Cartographer can skip artifact downloads for some reasons when constructing a dependency graph. This can happen if there is a network problem, if the machine is overloaded and there is a timeout etc. Then there are some subgraphs missing in the resulting repository. So after a dependency graph we can request the list of incomplete artifacts from Indy/Cartographer and check if they are really not available. If some of them are, we should fail the repo build.