Post comment on PR/MR if pipelinerun can't be started for some reason

[ralphbean]

We have a scenario where users are submitting merge requests to a gitlab repo (correctly enrolled with PaC) from a private fork that PaC doesn't have access to with its token. PaC sees the merge request event, but cannot access the private fork to retrieve the pipelienrun definition - and it fails. It will emit errors in its controller logs about this, but the end user doesn't see this. It looks like silence from PaC in the merge request itself.

:bulb: When PaC cannot start a pipelinerun on a mergerequest or pull request event for some reason like this, it should comment on the PR/MR with the reason. Even if not much useful information can be provided, it is better than radio silence.

There are some cases (like evaluation of on-cel-expression that precludes the execution of the pipelinerun) where a comment on the PR/MR is not desirable.