Added expect error message to TimeFeedback where there used to be an unwrap (#2777)

* Added expect error message to TimeFeedback where there used to be an unwrap

* Changed error message handling

* fixed cargo clippy

---------

Co-authored-by: Dominik Maier <domenukk@gmail.com>

libafl/src/feedbacks/mod.rs

@@ -932,7 +932,12 @@ where
         observers: &OT,
         testcase: &mut Testcase<I>,
     ) -> Result<(), Error> {
-        let observer = observers.get(&self.observer_handle).unwrap();
+        let Some(observer) = observers.get(&self.observer_handle) else {
+            return Err(Error::illegal_state(
+                "Observer referenced by TimeFeedback is not found in observers given to the fuzzer",
+            ));
+        };
+
         *testcase.exec_time_mut() = *observer.last_runtime();
         Ok(())
     }