fix: Reject instead of returning a null table

- Returning a `null` instead of an actual table is weird. Throw to indicate the key could not be found.

web/client-api/src/main/java/io/deephaven/web/client/api/JsPartitionedTable.java

@@ -169,8 +169,8 @@ public Promise<JsTable> getTable(Object key) {
         }
         final List<Object> keyList = Js.<JsArray<Object>>uncheckedCast(key).asList();
         if (!knownKeys.contains(keyList)) {
-            // key doesn't even exist, just hand back a null table
-            return Promise.resolve((JsTable) null);
+            // key doesn't even exist
+            return Promise.reject("Key not found");
         }
         final String[] columnNames = descriptor.getKeyColumnNamesList().asArray(new String[0]);
         final String[] columnTypes = keyColumnTypes.toArray(new String[0]);