The images were dropped by @Wimpy 4 days ago. The release reaches end of life at the end of the month.
You can still download it via BitTorrent using the magnet link, providing there are seeders still about. It can be opened in Transmission using the File → Open URL option.
magnet:?xt=urn:btih:01fa02661144c6a26ff09cb7a9bceb1c48716972&dn=ubuntu-mate-15.10.3-desktop-armhf-raspberry-pi-2.img.xz
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