Define error code for failed factorization

[mtanneau]

Currently, failed factorizations are handled in the IPM solver via a try ... catch block that intercepts PosDefExceptions and increases regularizations.

It would be more flexible/general to have KKT.update! return a status code, which would be used to flag numerical issues, etc. This would allow to do away with the try ... catch, which is not the most compiler-friendly approach.