GitLab hooks: removed annoying and misleading failure message

The original wording was appropriate when the pre-receive call was
done actually after the transaction. It was then useful to warn the
user of a possible decrepancy. But an error in pre-receive has long
since become a trigger for transaction abort.

On the other hand, the original message was still appropriate for
the post-receive call, expanding it a bit to include "actions" not
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