Render an Error on the Billing Page When CustomersDot is Unavailable

What does this MR do?

Display an error page when CustomersDot is down and the user visits the Billing page.

Previously, this scenario would result in a 500 error from GitLab.

This MR fixes this issue for both the Group and Profile billing pages.

Issue: #320931 (closed)

UX: #320931[Billing_page_error_loading.png]

Screenshots

Group Billing Page When CustomersDot is Available:

This contains no changes and is just here for context.

Group Billing Page When CustomersDot is Unavailable:

This is one of the changes in this MR. Previously this page results in an error when CustomersDot is unavailable. See the last picture below.

Profile Billing Page When CustomersDot is Available:

This contains no changes and is just here for context.

Profile Billing Page When CustomersDot is Unavailable:

This is one of the changes in this MR. Previously this page results in an error when CustomersDot is unavailable. See the last picture below.

Billing Page When CustomersDot is Unavailable Prior to this MR:

This is a local screenshot. It would look different in production, but this illustrates the error. This is the behavior fixed by this MR.

Does this MR meet the acceptance criteria?

Conformity

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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