new AJAX responses not yet bubbling up to alerts for bulk re-indexes.

[tw2113]

In 2.11.0, we amended errors to be returned via wp_send_json_error instead of attempting to just echo out.

However, the javascript that receives that response has not been properly updated to receive that JSON error data and actually display the response data

See: https://github.com/WebDevStudios/wp-search-with-algolia/blob/2.11.1/includes/admin/js/reindex-button.js

While we're at it, let's make sure that the push settings buttons are/aren't affected and update appropriately. https://github.com/WebDevStudios/wp-search-with-algolia/blob/2.11.1/includes/admin/js/push-settings-button.js

Bonus points if we can remove jQuery based js as well and modernize the js as a whole. See https://github.com/WebDevStudios/wp-search-with-algolia/issues/431

[tw2113]

For the moment, I'm not completely de-jQuerying but am adding strict mode and small details.

PR can be found at https://github.com/WebDevStudios/wp-search-with-algolia/pull/534