Motivation for the change, related issues

The new Comlink error handler rewired the logic around passing errors from web workers to the Playground web app. Specifically, it enriched the error with additional stack trace before rethrowing it:

throw new Error('Comlink method call failed', {
	cause: originalError
});

Unfortunately, this also put the original error at the bottom of the error.cause chain, making error handling confusing for the API consumers.

This PR ensures the thrown error the original one. This way a simple try {} catch(e) {} is enough to log the error details without having to reason about e.cause.cause etc. The additional stack trace information is now attached at the bottom of the cause chain for when the developer needs to inspect it.

Testing Instructions (or ideally a Blueprint)

Run Playground with this Blueprint and confirm the top-level error message speaks about PHP failure. Specifically, it should not say "Comlink method call failed":

{
  "steps": [
    {
      "step": "mkdir",
      "path": "/wordpress/wp-content/mu-plugins"
    },
    {
      "step": "writeFile",
      "path": "/wordpress/wp-content/mu-plugins/test.php",
      "data": "<?php undefined_function();"
    }
  ]
}