throw clear error about unavailable db nodes

src/MariaDB.ts

@@ -337,6 +337,9 @@ class MariaDB
             const pool = this.#pool;
             if (nodeName)
             {
+                const available = this.#nodes.filter(n => n.status === 'synced');
+                if (!available.length)
+                    throw new Error('No nodes available for query');
                 const node = this.#nodes.find(n => n.name === nodeName);
                 if (!node)
                 {