Improve error message if socket isn't configured for TLS

src/ServerSocket.php

@@ -13,6 +13,15 @@ public function enableCrypto(): Promise {
             return new Failure(new ClosedException("The socket has been closed"));
         }
 
+        $ctx = \stream_context_get_options($resource);
+        if (empty($ctx['ssl'])) {
+            return new Failure(new SocketException(
+                "Can't enable TLS without configuration. " .
+                "If you used Amp\\Socket\\listen(), be sure to pass a ServerTlsContext as third argument, " .
+                "otherwise set the 'ssl' context option to the PHP stream resource."
+            ));
+        }
+
         return Internal\enableCrypto($resource, [], true);
     }
 }