Update simulation.py to always exit with code 0 assuming the simulation exits normally. If the running application has a return code that is non zero, then print the return code before exiting. Change-Id: I1983985d50311627574d4364b32ee961ae88e003 Signed-off-by: Jason Lowe-Power <jason@lowepower.com> Reviewed-on: https://gem5-review.googlesource.com/4880