Fix cobertura_report collection error because of earlier error, before tests

CoberturaPublisher is done at the end of the execution because it cannot be duplicated (at least it was the case several years ago when it was plugged-in).

With 1f3c17db399c85e923e5506e1c3d33636473bb57 / #482 we tried to eagerly collect tests results even when tests failed, but didn't properly handle that late collection in the case of errors before any test step: the tests results won't exist.

Because it's a global, late collection, we cannot do the fancy double collection with/without error propagation that we did in #482 for the other tests results collections. That's OK; the situation is explained to the user.

Deepomatic / dmake

Fix cobertura_report collection error because of earlier error, before tests #487