Fix error handling when pattern creation fails - #85117

What?

Closes #85089.

Display the original REST API error when pattern creation fails.

Why?

A failed pattern save currently returns undefined, which is passed to the success callback. This causes a JavaScript error about wp_pattern_sync_status, hiding the server’s error message.

How?

Pass { throwOnError: true } to saveEntityRecord in createPattern, allowing the modal’s existing error handler to display the original error.

Testing Instructions

1. Add this code to a temporary must-use plugin:

   <?php
   add_filter(
       'rest_pre_insert_wp_block',
       function () {
           return new WP_Error(
               'pattern_creation_failed',
               'Pattern creation was blocked for testing.',
               array( 'status' => 400 )
           );
       },
       10,
       0
   );

2. Open a post or page and insert a Paragraph block.
3. Open the block options menu and select Create pattern.
4. Enter a name and submit.
5. Confirm the snackbar displays “Pattern creation was blocked for testing.” instead of a JavaScript error.
6. Remove the temporary plugin and confirm pattern creation succeeds.

Use of AI Tools

OpenAI Codex was used to investigate the issue, PR Description.