Return refresh() result

[n7studios]

Summary

Updates the ConvertKit_Resource_V4 class to:

• Return the result of $this->refresh() in init()
• Return a WP_Error when refresh() is called without an API client being specified

This allows the Plugins calling this class to decide how to handle the WP_Error.

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)