Surface Zuora lock error

What does this MR do and why?

Issue: https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/7168

This displays a more specific error message when we hit a Zuora lock instead of a generic error message.

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Before	After

How to set up and validate locally

You'll need CustomersDot setup and properly configured.

1. [Cdot] add raise IronBank::UnprocessableEntityError.new(errors: [{code: 58730050, message: 'Operation failed due to a lock competition, please retry later.'}]) to Zuora::Orders::Preview#perform to simulate a Zuora lock error
2. [Gitlab] Create a group
3. [Gitlab] On the group page, go to settings > billing
4. [Gitlab] Click "upgrade to premium"
5. [Gitlab] You should see a "Operation failed due to a lock competition, please retry later." error