ci: add check for missing dependencies in vendor-check

[thaJeztah]

relates to #2329

vndr prints missing dependencies as a WARNING. We currently don't appear to be catching these issues, and ignore those warnings.

We should update the vendor check to fail if a WARNING: dependency is not vendored is printed;

vndr github.com/Microsoft/hcsshim

2020/02/12 16:40:16 Collecting initial packages
2020/02/12 16:40:16 Download dependencies
2020/02/12 16:40:20 	Clone github.com/Microsoft/hcsshim, revision b3f49c06ffaeef24d09c6c08ec8ec8425a0303e2, attempt 1/20
2020/02/12 16:40:25 	Finished clone github.com/Microsoft/hcsshim
2020/02/12 16:40:25 Dependencies downloaded. Download time: 4.625264419s
2020/02/12 16:40:25 Collecting all dependencies
2020/02/12 16:40:32 WARNING: dependency is not vendored: github.com/containerd/cgroups/stats/v1
2020/02/12 16:40:32 Clean vendor dir from unused packages
2020/02/12 16:40:32 Success
2020/02/12 16:40:32 Running time: 16.278317994s

[Madhur1997]

I can take this up.

[thaJeztah]

Thanks!