Allow configurable "fail when no merge"

[ssorallen]

It would be great to allow configuring "fail when no merge was performed." The action always calls exit 0 when no merge is performed, but I would like to make that exit 1 to stop subsequent actions from running.

https://github.com/robotology/gh-action-nightly-merge/blob/81570ba03dd370f582bd3f52d47672d29191829f/entrypoint.sh#L60-L63

My use case, I am using this action to run a nightly build:

1. this action runs to merge main into a named branch
2. then another action is called to kick off an iOS build + deploy

I would like to not run step 2. if no merge was performed.

[traversaro]

It make sense to me, put probably we want to have this under an option for back compatibility. Would you be interested in preparing a PR? Thanks!