Gracefully handle unavailability / errors in GitLab IDP

MR: Pending

Description

Based on this production incident:

• gitlab-com/gl-infra/production#18920 (closed)
• https://gitlab.slack.com/archives/C0837PP1EHY/p1733162588860589

Scenario: As a GitLab Web IDE user, when the GitLab IDP (identity provider) is not functional (for example due to a Google Cloud outage) I'd like the error to be gracefully handled.

Acceptance Criteria

TODO: Fill out (required)

• Graceful error handling from the user's perspective
• Tests to ensure this scenario is covered