fix: call config.onError if present if isReadyToPay throws

[axelniklasson]

Currently if the isReadyToPay throws when setting up the Google Pay button, the onError callback is not called and the error is printed out in console through console.error instead. This PR adds support for triggering the onError callback if present and otherwise fallback to the existing behaviour.

[socsieng]

Thanks for the PR. Can you include repro steps you used to trigger the error?

[axelniklasson]

Sure!

1. Serve any page embedding the React button locally over an insecure context (such as your local IP)
2. When rendering it crashes and the error message DEVELOPER_ERROR in isReadyToPay: Google Pay APIs should be called in secure context! is printed in the console
3. The onError callback of the React component is not triggered