Signing-up for GitLab: Audit error messages
## Problem
Some Error messages are duplicated, other are inconsistent, both in terms of tone and messaging as well as placement. This is a really quirky experience and is not helping user efficiently recover from their error.

## Input data
* [Poor error handling when user is signing up and selects an existing group name](https://gitlab.com/gitlab-org/gitlab/-/issues/331973)
* [Pajamas form validation guidelines](https://design.gitlab.com/components/form#errors)
* [Content > Error messages guidelines](https://design.gitlab.com/content/error-messages/)
## Solution
Audit all error messages in the signing up flow and ensure the following:
* They comply with our Pajamas guidelines.
* The copy complies with our guidelines.
Create subsequent issue to fix the errors.
issue