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.
Fixed bug for QM when no thermo found for radical.