clarify what file has to be created

proxy/src/main/java/com/velocitypowered/proxy/config/VelocityConfiguration.java

@@ -499,7 +499,7 @@ public class VelocityConfiguration implements ProxyConfig {
                 "The file " + forwardSecretFile + " is not a valid file or it is a directory.");
           }
         } else {
-          throw new RuntimeException("The forwarding-secret-file does not exist.");
+          throw new RuntimeException("The file '" + secretPath + "' does not exist.");
         }
       }
     }