Improve plugin center error feedback and cache invalidation (#2147)

The plugin center cache was not invalidated when the proxy configuration was changed in the global settings. This caused stale and inconsistent state to be displayed to the user while there was no feedback that something was wrong.

scm-ui/ui-api/src/plugins.test.ts

@@ -114,6 +114,7 @@ describe("Test plugin hooks", () => {
     _embedded: {
       plugins,
     },
+    pluginCenterStatus: "OK",
   });
 
   const createPendingPlugins = (