POC: Provide a redirect URL for OAuth errors

[techanvil]

Summary

Add PoC support for an error redirect URL for OAuth errors.

This allows the default behaviour which is to redirect to the dashboard or splash screen to be overridden, e.g. to allow the user to return to the Settings screen on an OAuth error.

Note that the error redirect URL is not supported for the initial site setup as it's not needed there, it's only available for subsequent OAuth connections e.g. to provide additional scopes.

Relates to issue #8179.

Relevant technical choices

PR Author Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 5.2 and PHP 7.4.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have added a QA Brief on the issue linked above.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

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Code Reviewer Checklist

• Run the code.
• Ensure the acceptance criteria are satisfied.
• Reassess the implementation with the IB.
• Ensure no unrelated changes are included.
• Ensure CI checks pass.
• Check Storybook where applicable.
• Ensure there is a QA Brief.

Merge Reviewer Checklist

• Ensure the PR has the correct target branch.
• Double-check that the PR is okay to be merged.
• Ensure the corresponding issue has a ZenHub release assigned.
• Add a changelog message to the issue.