Fatal error handling

[jimmielin]

In geoschem_src in GeosUtil/error_mod.F90 there are some error handlers

~/devel/geos-chem/GeosUtil/error_mod.F90:
  501  
  502  #elif defined( MODEL_CESM )
  503:       CALL ENDRUN('GEOS-Chem failure!')
  504  
  505  #else

~/devel/geos-chem/Headers/errcode_mod.F90:
  142  
  143  #if defined( MODEL_CESM )
  144:     CALL ENDRUN('GEOS-Chem failure!')
  145  #endif
  146  

which could probably point to cesm.log to see the actual GEOS-Chem error messages if they're not caught upstream by chemistry.F90, for user friendliness. I will make a PR to the GC main repo later, just noting this down.