[pixiv] Make gallery-dl download alternate file when original unavailable

[NonaSuomi]

It appears that for some reason certain images are unavailable in their original resolution, resulting in HTTP 500 error responses from the server.

Example: https://www.pixiv.net/artworks/56360615
Page 3 of the linked work is unavailable at https://i.pximg.net/img-original/img/2016/04/15/11/53/44/56360615_p3.jpg
However, there is a version still up at https://i.pximg.net/img-master/img/2016/04/15/11/53/44/56360615_p3_master1200.jpg

Is there some way to make GDL recognize this, perhaps after the retries threshold is exceeded, and attempt to download the resized version instead of simply skipping them entirely?