Previously, when trying to upload the build artifacts for a PR/SHA that was
already successfully deployed (e.g. when re-running a Travis job), the preview
server would return a 403 and the build would fail.
Since we have other mechanisms to verify that the PR author is trusted and the
artifacts do indeed come from the specified PR and since the new artifacts
should be the same with the already deployed ones (same SHA), there is no reason
to fail the build. The preview server will reject the request with a special
HTTP status code (409 - Conflict), which the `deploy-preview` script will
recognize and exit with 0.
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