Duplicate registrations cause internal server error instead of being gracefully reported

[Muirrum]

[Muirrum]

https://github.com/wpi-acm/Hack-WPI-Python/blob/master/goathacks/registration/__init__.py#L33

Should check for existing email here before continuing to check passwords. If email already, exists, redirect to login page

[ngolp]

See branch handle-duplicate-registration