settings-committer: INFO instead of error when no settings changed

[tjkirch]

On reboot, for example, we may not have had any settings change. We don't want
that to cause a service startup failure or log a confusing message. If other
services failed to PATCH settings changes, they'll fail themselves, so the
system will still reflect failure when it needs to.

---

Testing done:

Before, after a reboot, settings-committer would fail and show its error on console:

Nov 22 17:09:43 ip-192-168-111-223.us-west-2.compute.internal systemd[1]: Starting Commit pending settings...
Nov 22 17:09:43 ip-192-168-111-223.us-west-2.compute.internal settings-committer[3131]: Error sending POST to /settings/commit: Status 422 when requesting unix://2f72756e2f6170692e736f636b:0/settings/commit: Tried to commit with no pending changes
Nov 22 17:09:43 ip-192-168-111-223.us-west-2.compute.internal systemd[1]: settings-committer.service: Main process exited, code=exited, status=1/FAILURE
Nov 22 17:09:43 ip-192-168-111-223.us-west-2.compute.internal systemd[1]: settings-committer.service: Failed with result 'exit-code'.
Nov 22 17:09:43 ip-192-168-111-223.us-west-2.compute.internal systemd[1]: Failed to start Commit pending settings.

bash-5.0# systemctl status | head                                                                                                                                                  
● ip-192-168-111-223.us-west-2.compute.internal
    State: degraded

After, no error:

bash-5.0# journalctl -u settings-committer
-- Logs begin at Fri 2019-11-22 18:08:52 UTC, end at Fri 2019-11-22 18:10:46 UTC. --
Nov 22 18:09:27 ip-192-168-119-189.us-west-2.compute.internal systemd[1]: Starting Commit pending settings...
Nov 22 18:09:27 ip-192-168-119-189.us-west-2.compute.internal systemd[1]: Started Commit pending settings.

bash-5.0# systemctl status | head
● ip-192-168-119-189.us-west-2.compute.internal
    State: running

Failures still cause the system to... fail, as expected.