Fail gracefully if solr not enabled

Right now, if solr is unavailable in the pantheon environment (such as when you switch a project to a Basic plan that does not include solr), both the search pages and search api config page error with a white screen that says:

Notice: Undefined index: PANTHEON_INDEX_HOST in /srv/bindings/XXXXXXXXX/code/web/modules/contrib/search_api_pantheon/src/Plugin/SolrConnector/PantheonSolrConnector.php on line 69

The module should be able to fail more gracefully if solr isn't available, and at a minimum not error with a white screen. Ideally, a message would be displayed saying that solr isn't currently available for the pantheon environment and to check that it is enabled and your site level supports solr integration.