Cancel message for "Add To Merge Train When Pipeline Succeeds" auto merge strategy is not descriptive enough

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The following discussion from !13767 (merged) should be addressed:

• @ayufan started a discussion:

  I think that this message is not descriptive enough when cancel happens due to MWPS unable to enqueue to train.

  We should have a better message with better reason, like: merge is no longer mergable, failed to add to mege train.

  Maybe this should be returned by MergeTrainService#execute result and be passed to notice send to the system.