🐛 Enhance and surface imposter commit errors.

[spencerschrock]

Follow up of #389. The original PR didn't wrap the error in the new verification error which was introduced in #408.

So when a workflow was rejected for having an imposter commit, the user got this message:

error processing signature: http response 500, status: 500 Internal Server Error, error: {"code":500,"message":"something went wrong and we are looking into it.

This PR wraps the imposter commit message so the error is surfaced. The error message also now includes the ref which triggered the warning, so the user doesn't need to guess/check.

error processing signature: http response 400, status: 400 Bad Request, error: {"code":400,"message":"workflow verification failed: imposter commit: deadbeef088af7c0dc8046f3ac9a8f4148492a95 does not belong to step-security/harden-runner, see https://github.com/ossf/scorecard-action#workflow-restrictions for details."}

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