When the test class isn't annotated, the execution of Warp will throw NullPointerException:

https://github.com/arquillian/arquillian-extension-warp/blob/1.0.0.Alpha2/api/src/main/java/org/jboss/arquillian/warp/Warp.java#L37

Provide a meaningful message which will instruct a user to add @WarpTest annotation.