Fix label association and error handling in user override select

What does this MR do and why?

Follow-up fixes from the review of !253351 (merged), addressing the three deferred discussions tracked in https://gitlab.com/gitlab-org/gitlab/-/issues/628085

References

• Closes https://gitlab.com/gitlab-org/gitlab/-/issues/628085
• Follow-up from !253351 (merged)

Screenshots or screen recordings

The only visible UI change is a danger GlAlert that appears above the user select when a search request fails (previously the failure was silent). Screenshots could not be captured in this development environment; the error state can be viewed in the added WithSearchError Storybook story instead.

No error	Error

How to set up and validate locally

1. Run the specs:

   yarn jest ee/spec/frontend/usage_quotas/wallet_agnostic_credits_dashboard/credit_caps

2. Run Storybook with yarn storybook, open EE / Usage quotas / Wallet agnostic credits dashboard / Credit caps / User override select, and check the Default and WithSearchError stories.
3. On the GitLab Credits dashboard, open the Add override modal and confirm clicking the Users label focuses the token selector input (label/for association now resolves to the input's id).

MR acceptance checklist

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