Add another error message to SAML troubleshooting section

What does this MR do and why?

Adding a new error message to SAML troubleshooting section, as the error occurred for several customers

Screenshots or screen recordings

How to set up and validate locally

1. Have an account provisioned by SCIM, and don't reset password
2. Unlink that account
3. Attempt to relink account

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.