I do not quite understand what is causing the grub.cfg to not exist.
The logic behind this code:
for path in [self.update_grub_command,
self.grub_install_command,
self.config_file,
self.header_config_file]:
if not os.path.isfile(path):
raise SystemExit('Fatal error. Can not find %s' % path)
# Make sure config files are up to date
os.system(self.update_grub_command)
Is that it does not check for grub.cfg because it may not exist at the time,
but is supposed to be created by update-grub, which is run immediately after
the check.
The first line of output you provided is not from sum, but it may be
something that blocks update-grub from working. Try to investigate
/boot/grub/device.map
