don't fail if promotion is unrequired

[ynohat]

Currently pipeline promote fails with a non-zero exit code if the promoted version is the same as the deployed version.

I'd prefer it to be a no-op with a 0 exit code.

Consider this scenario as an example where this is a bug:

I have a repository with the pipeline configuration and some scripts used by my CI tool. The CI tool triggers a build which promotes to successive envs when a new version is pushed.
If I make a change to the CI scripts, I expect the builds to succeed, but they'll fail because there was no change.