Add Assertions on CS Application in Snapshot Logic (#58681) (#59511)

Relates to #58680. Bugs like that should not only show up in logs
but ideally also get caught in tests. We expect to never see exceptions
in these two spots.

server/src/main/java/org/elasticsearch/repositories/RepositoriesService.java

@@ -355,6 +355,7 @@ public class RepositoriesService extends AbstractLifecycleComponent implements C
             }
             repositories = Collections.unmodifiableMap(builder);
         } catch (Exception ex) {
+            assert false : new AssertionError(ex);
             logger.warn("failure updating cluster state ", ex);
         }
     }

server/src/main/java/org/elasticsearch/snapshots/SnapshotShardsService.java

@@ -137,6 +137,7 @@ public class SnapshotShardsService extends AbstractLifecycleComponent implements
             }
 
         } catch (Exception e) {
+            assert false : new AssertionError(e);
             logger.warn("Failed to update snapshot state ", e);
         }
     }