Modify a false positive GRBException

When check the schedulability of a taskset, the constraint solver may
not be able to sovle the model so that it will not have the objective
value; When in writeDetailedOutput() try to get this value, it will
raise GRBException error. However ,this error is caught and handled in
the programs. So no need to print out the exeption's stack, which will
confuse users that the program is not behaving correctly, althought it
does.