Rethrow uncaughtException when joining a Fiber

Author: eregon

src/main/java/org/truffleruby/core/fiber/FiberManager.java

@@ -350,6 +350,11 @@ private void doKillOtherFibers() {
                     finishedLatch.await();
                     return BlockingAction.SUCCESS;
                 });
+
+                final Throwable uncaughtException = fiber.uncaughtException;
+                if (uncaughtException != null) {
+                    ExceptionOperations.rethrow(uncaughtException);
+                }
             }
         }
     }