fix: throw an error when login() or logoff() is unexpected

src/Client.js

@@ -55,6 +55,10 @@ export class Client extends EventEmitter {
     }
 
     login({ seed }) {
+        if (this.#ws && this.#ws.readyState === WebSocket.OPEN) {
+            throw new Error('already connected')
+        }
+
         const ws = new WebSocket(this.#gateway)
         this.#ws = ws
 
@@ -81,6 +85,10 @@ export class Client extends EventEmitter {
     }
 
     logoff() {
+        if (!this.#ws || this.#ws.readyState !== WebSocket.OPEN) {
+            throw new Error('not connected')
+        }
+
         this.#ws.close()
     }