Neglecting to solve the captcha on sign up yields a blank white page for the end user

If a user fails to solve the captcha on sign up, they will be presented with a blank white page after submitting the form. User should be returned to the sign in/up page with an appropriate error message.

Steps to reproduce:

1. Go to GitLab.com
2. Fill out the sign up form, completing all fields EXCEPT the captcha.
3. Click the sign up button.
4. Observe the white page.

cc/ @DouweM Please weigh/assign.