Provide better message for bad tests/env
Provide better message for bad tests/env

We provide a nice error message when you `make run` with a bad
local.env, but `make test` with a bad tests/env was giving just
SystemExit. This commit subclasses SystemExit to provide a more
specific error message.

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