Output unresolved dependencies

What does this MR do?

This MR changes the error returned when a project fails to build due to unresolved dependencies. The previous code would simply output an error message explaining that unresolved dependencies exist, this MR changes this to output a list of the unresolved dependencies.

Before:

Project has unresolved dependencies

After:

Project has 2 unresolved dependencies: fluff:invalid:1.0.10, fuzz:broken:2.1.20

What are the relevant issue numbers?

gitlab-org/gitlab#346992 (closed)