Exclude special generators from certain logParser tests

Changes introduced

• Introduce list specialGenerators in logParser to allow to skip certain tests that are not available e.g. for HI generators (like cross-section, or TestHepMC)
• If TestHepMC results are missing for Sherpa throw ERROR instead of WARNING (@cgutscho I am not sure why we had this only as a warning instead - can change it back if we don't want this behaviour)

Tests

From the logs provided in #66 (closed)

Before

With the changes in this MR

Issues resolved

Closes #66 (closed)