MG download link on study page fails for screened data

[wltrimbl]

The study page has download links in the table of metagenomes.

These buttons on
https://www.mg-rast.org/mgmain.html?mgpage=project&project=mgp13068
make calls like
https://api-ui.mg-rast.org/download/mgm4624578.3?file=050.1&link=1
which fails with HTTP 403 for some datasets because of h.sapiens screening.

Either

1. API needs to give a link to 299 though it's misleading,
2. MGRASTv4 needs to give a better error message or
3. Add a button for 299 to the project page