Code review comment for lp:~jtv/maas/custom-dhcp

Good questions — and luckily I provided answers in my predecessor branch! In that preparatory branch I extracted the sudo_write_file helper, and in view of Raphaël's painful debugging experience, made it check sudo's return code and report sudo's error message. I haven't looked into how that message gets displayed, really, so there may be separate work to be done on that.

I see some value in bundling up the rewriting of files, but there isn't much to bundle anyway. Plus it's all done synchronously to service restarts, so arguably the real value would be in ensuring that we retry on failure.