Call extract_decays from check_sanity

check_sanity should call extract_decays to make sure the workfile is
up-to-date
remove print from etract_decays on success
On error do not return but raise SystemExit

changed return statements to raise SystemExit and added error-codes
according to https://docs.python.org/2.6/library/os.html#os.EX_OK
Not sure if those error-codes can be unittested or if it would be better
to give a unique number to each exit on error