fix(coding-conventions): Refactor relocation API call to use try/catch

[sentry]

This PR addresses a false positive reported by the no-callback-api-request convention checker in static/gsAdmin/views/relocationCreate.tsx.

The original issue was that the checker incorrectly flagged line 64 as a violation, even though the LLM analysis correctly identified the code snippet at that line as compliant. The actual violation was a requestPromise().catch() chain at lines 48-53, which the checker was misreporting.

This fix refactors the promoCodeApi.requestPromise().catch() pattern into a try/catch block. This brings the code into full compliance with the no-callback-api-request convention and removes a hacky throw new Error() workaround, replacing it with a clean return statement for the specific 403 error case.

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Fixes CODING-CONVENTIONS-6MF

_{@sentry <feedback>: Autofix iterates on these changes
@sentry stop iterating: Autofix stops iterating on this run}