Make release script more robust and send user agent

[phil-opp]

The data access policy of crates.io requires a user-agent header that identifies the application. Otherwise it may block the request. See https://crates.io/data-access for details.

This commit adds the required user-agent header and also fixes a related bug that misclassified the error response as 'release does not exist yet'. We now only create a new release if we're sure that the release does not exist yet. Errors are raised and fail the CI job so that we can investigate them.

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Discovered in https://github.com/rust-osdev/bootloader/actions/runs/30249893235/job/89925267279

gh: Validation Failed (HTTP 422)
{"message":"Validation Failed","errors":[{"resource":"Release","code":"already_exists","field":"tag_name"}],"documentation_url":"https://docs.github.com/rest/releases/releases#create-a-release","status":"422"}Detected crate version 0.11.16
Could not find version 0.11.16 on crates.io; creating a new release

The API returned an error here and the script misclassified it as 'no release exists' and tried to publish.

[Freax13]

LGTM, thanks!