If QM fails for 1 resonance isomer, use the one that works
Better way to fix the resonance isomer QM problem
Debug help when parsing mopac log files.

Trying to help issue #134.
This is unlikely to fix anything, but should help debugging.
Write the QM thermo entry to libray only if QM calculation was successful.
Do not fail when QM attempt is unsuccessful in Gaussian.

Return None for thermo so that that it can fall back to group additivity, same
as for Mopac.