cylc task logging: "Send message 1 of 7 succeeded" bugfix

#816 introduced a change that was intended to suppress the `Send message 1 of 7 succeeded` line that appears in task output, by only making it visible once a send message attempt failed - i.e. `2 of 7 succeeded` or higher.

Long story short: the logic needs `> 1`!

@hjoliver, please review.