src/grd: use sys.exit
The exit function is a constant from the site module, which "should not
be used in programs". Replace it with sys.exit()
Related: https://docs.python.org/3/library/constants.html#constants-added-by-the-site-module
Change-Id: I95db013e36242d5126ce07b8a392e6dc0a0ecfdd
