Proof-of-concept for top-level errors in CI mode

As discussed in  stefanpenner/ember-cli#2142 and stefanpenner/ember-cli#2205 , if something goes very wrong in CI mode, tests just don't run instead of reporting the error(s).

As I'm not really familiar with this codebase yet, here's a small proof-of-concept on what I'd like to achieve.. If this approach is ok for you, I'll add test(s) to this PR so it can get merged.