covariance quality artificially set to -1 if inversion covMat to hessian fails
In some cases, at (1) the covQual of the RooFitResult is artificially set to -1 (covMat not available) even if the covMat itself was before reported as status 3 (fully accurate). In the case that I ran into this issue the 'hesse' member of the RooFitUtils::ExtendedMinimizer::Result pointer 'r' was simply not set. The reason for it not being set was that the inversion from the (well existing) covMat failed due a 0 or nan determinant. This, however, can easily be due to machine accuracy in the inversion process. If RooFit reports that the (initially provided) covMat is accurate this seems like a very likely case and we shouldn't claim it wasn't even available.
NB: this modification of the status code, for example, also causes CAFCore's SFramework to not even try extracting the covMat itself again.
(1) https://gitlab.cern.ch/cburgard/RooFitUtils/blob/master/Root/ExtendedMinimizer.cxx#L832