when no renewal is done, should return a different exit code

[pdumais]

I am running lego using the goacme/lego docker image.
I am setting a cronjob to renew my certificates every day and if a renewal actually takes place, I need to restart my webserver. I don't want to blindly restart the webserver if no renewal took place (because there was still a good amount of days left until renewal).

For this reason, I think it would be very useful if lego would return a non-zero exit code if it refused to do the renewal.

[ldez]

Related to #216 (comment) and #427

[pdumais]

If you prefer not to use an exit code, then may I suggest adding a flag that will instruct lego to return, on stdout, a predictable short string that says "updated" or "noop" ?