Move test for undefined socket closer to usage

We found that if sendMessage() was called but not awaited on, the
code execution would get to the awaited signRequest() call and then
immediately return execution to caller of sendMessage() before
signRequest() returns. If the caller then calls disconnect(), when
signRequest() returns, the socket would be undefined, even though
our test for undefined above would find it existing.

src/client/WebSocketClient.ts

@@ -481,10 +481,6 @@ export class WebSocketClient extends EventEmitter {
   }
 
   async sendMessage(message: WebSocketRequest): Promise<void> {
-    if (!this.socket) {
-      throw new Error(`Failed to send message of type "${message.type}": You need to connect to the WebSocket first.`);
-    }
-
     /**
      * Authentication will result in a couple of benefits:
      * 1. Messages where you're one of the parties are expanded and have more useful fields
@@ -498,6 +494,10 @@ export class WebSocketClient extends EventEmitter {
     });
     Object.assign(message, signature);
 
+    if (!this.socket) {
+      throw new Error(`Failed to send message of type "${message.type}": You need to connect to the WebSocket first.`);
+    }
+
     this.socket.send(JSON.stringify(message));
   }