I second that this is most likely a corrupt .iso. I'm actually currently fighting through what appears to be an Akamai (content delivery network) node with a bad copy stored, so until I can get it to expire or assign a new node for download I'm stuck. Note that even if you're a volume licensing customer and not MSDN, the SHA1 hashes are still available to view on the MSDN site even if the downloads are not accessible (for some unknown reason they're missing from the licensing download site).
For the ...x15-59754.iso, the SHA1 is
For the ...x15-50365.iso, the SHA1 is