contenttest: Exit with non-zero status when tests fail

This makes buildbot reject commits with failing contenttests.
auto merge of #1130 : kmcallister/servo/contenttest, r=metajack

**This build is expected to fail** at first, because `test_img_width_height` is broken on `master`. It's the simplest way I know to check that with this change, buildbot will indeed catch failing content tests.

After that happens I'll push the commits to fix `test_img_width_height` and request a re-review.