Return nonzero exit code on STOP

Sometimes I see CONEX running into a situation when STOP is called, which prints some error message to stderr and ends the running process. Unfortunately, this still results in an exit code of 0, meaning a successful completion of the program. I would prefer to have a non-zero exit code to make it easier to detect errorneous runs. You could do that by calling STOP 3 (or any other non-zero number < 128). While it is not required by the FORTRAN standard, at least gfortran and ifort use the number as exit code.