Application manager->tools->application catalog->
Then look at 'Details' for each of your entries (or maybe you know exactly which one it is) until you find the one with that URL (it's for Canola).
Then tick 'Deactivate' (or 'Disable' - I'm not sure what's in your locale).
Then 'close'. It'll update the list for you again, this time without trying that failed repository.
The W: can be ignored, but it looks like you have the same repo added twice to your application catalog.