threadpool: add panic_handler (#1052)

tokio-threadpool/src/task/mod.rs

@@ -165,6 +165,12 @@ impl Task {
                 // Transition to the completed state
                 self.state.store(State::Complete.into(), Release);
 
+                if let Err(panic_err) = res {
+                    if let Some(ref f) = unpark.pool.config.panic_handler {
+                        f(panic_err);
+                    }
+                }
+
                 Run::Complete
             }
             Ok(Ok(Async::NotReady)) => {