Migrate user-onboarding-*.spec.ts from Cypress to Playwright (#11927)

Co-authored-by: R Midhun Suresh <hi@midhun.dev>
Co-authored-by: Johannes Marbach <johannesm@element.io>
Co-authored-by: Milton Moura <miltonmoura@gmail.com>

playwright/plugins/synapse/index.ts

@@ -193,6 +193,10 @@ export class Synapse implements Homeserver, HomeserverInstance {
             },
         });
 
+        if (!res.ok()) {
+            throw await res.json();
+        }
+
         const data = await res.json();
         return {
             homeServer: data.home_server,