Translation:I came to tell you that I am going away.
Here is a website that might help: http://french.about.com/od/grammar/a/objectpronouns.htm
This phrase reminded me of a passage in St John's Gospel (14.28) "You have heard how I said unto you, I go away, and come again unto you"- 1611 translation. "Vous avez entendu que je vous ai dit: je m'en vais, et je reviens vers vous" -Trad French. "You heard me say to you, "I am leaving, but I will comeback to you".- 1976 translation. ".....dit,"je m'en vais, mais je reviendrai vers vous."- 1971 Fr. I don't know what Fr translations were published in the 17th century.
The reason for the simple present in that quote, as far as I can tell, is that it's habitual. "(Whenever) I go away, I come back." . So yes, "I came to tell you that I leave" could mean "just to level with you, I'm the kind of commitment-phobe who is habitually leaving with no warning." But it doesn't work as a way of informing someone that you are leaving in the present moment or near future, as the present continuous would.
Mick Harvey translated it as I have come to tell you I'm going for the title of his english cover of the Gainsbourg song and then translated it more freely in the actual song as I have come to tell you it's goodbye.
While the latter is perhaps too free a translation for our purposes here I think the former is a better and more natural translation than the one duolingo is currently showing.
It's the second translation that he uses in the song; the first he just uses as a more literal translation of the title. But you're right, to match the rhythm of the song made for the original french it needs ten syllables and I left out a word above. It's actually I have come to tell you that it's goodbye.
I think it's a good cover and worth a listen, but I'm wary of inserting youtube links into general discussion on duolingo. So I'll put the link in your personal stream.