Signing up for GitLab: Fix error message for existing email.
## Problem Our current error message for user that tries to register with an existing email: * Does not comply to our [error guidelines](https://design.gitlab.com/components/form#errors) * Is not suggesting known users to sign-in instead. ![image](/uploads/745b5adeea8c918ad3289360401fc5ab/image.png) ## Potential Solution Design TBC: ![image](/uploads/5b67e5ecc4eaf47275955bb5b354a31e/image.png)
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