Fix the output of put-mapping errors

[felipeelia]

Description of the Change

Currently, if the put-mapping operation returns an error, the message returned says Mapping failed: Array. This PR fixes that introducing a general function that gets an Elasticsearch response and returns the error reason in it (if any.)

How to test the Change

Add the following snippet to your codebase and run wp elasticpress sync --setup, wp elasticpress put-mapping, or sync via the UI:

add_filter(
	'ep_post_mapping',
	function ( $mapping ) {
		$mapping['mappings']['properties']['post_title']['index_options'] = 'something_wrong';
		return $mapping;
	}
);

Changelog Entry

Fixed - Display the error message returned by Elasticsearch if a mapping operation fails.

Credits

Props @felipeelia

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.