Do not throw error if unsupported payment gateway found

[dominik-zeglen]

I want to merge this change because it handles payment gateways that are unsupported simply by omitting them.

Pull Request Checklist

1. All visible strings are translated with proper context.
2. All data-formatting is locale-aware (dates, numbers, and so on).
3. The changes are tested.
4. The code is documented (docstrings, project documentation).
5. Changes are mentioned in the changelog.

Test Environment Config

API_URI=https://master.staging.saleor.rocks/graphql/