Can you try doing the same for Bitcoin Core 0.14.0 and 0.15.0 just in order to see whether this issue is limited to your end or a specific version? I've sent Wladimir a message and don't have OSX myself to test on it.
I tried these versions 0.15.0, 0.14.0,0.13.0, unfortunately there is a good signature found. I also tried downloading the dmg package and verified in window pc, it also failed. Here is a asc file I used(Sorry, I don't find a way to upload file), could you help to have a look at it in case I do something wrong when I copied it from the SHA256SUMs.asc file.
Here is the content of the file, and I use notpad++ to create this file:
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.11 (GNU/Linux)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=kBtV
-----END PGP SIGNATURE-----