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 1. Fill out the sign up form, completing all fields EXCEPT the captcha. 1. Click the sign up button. 1. Observe the white page. cc/ @DouweM Please weigh/assign.