Fail when platform is unknown; #482
Fail when platform is unknown; #482

There's a get_accounts_elsewhere function which, for now at least, is
hardwired for the two we currently support: GitHub and Twitter. We want
to hear about it if somehow we get another platform in there.

bountysource/www.gittip.com,eXcomm/gratipay.com,mccolgst/www.gittip.com,gratipay/gratipay.com,gratipay/gratipay.com,studio666/gratipay.com,studio666/gratipay.com,bountysource/www.gittip.com,studio666/gratipay.com,eXcomm/gratipay.com,gratipay/gratipay.com,bountysource/www.gittip.com,mccolgst/www.gittip.com,eXcomm/gratipay.com,bountysource/www.gittip.com,mccolgst/www.gittip.com,eXcomm/gratipay.com,mccolgst/www.gittip.com,studio666/gratipay.com,gratipay/gratipay.com