Improved error experience when creating a project

Previously, when users tried to connect their VCS with a GitHub account that was already linked to a separate CircleCI account, they would encounter a status 500 error. This would manifest in the CircleCI UI as an inability to list repositories on the form to create a project and the screen appeared frozen. Users that run into this scenario will now no longer see a frozen screen and instead be redirected to a page where they see an error message telling them remediation steps.