Include context in the vcs error message

[guywithnose]

What does this do / why do we need it?

Adds more context to vcs errors

What should your reviewer look out for in this PR?

When vcs fails, more information is shown. For example if you run dep init -v and a vcs command times out (like #832), more information is shown.
unable to update repository: becomes unable to update repository: command killed after 2m of no activity

Do you need help or clarification on anything?

Which issue(s) does this PR fix?

This doesn't fully fix, but it's related to #832.

[guywithnose]

Just realized I edited a file in the vendor folder. I'll submit to that repo.