exec concurrent gen: reacquire lock no matter what

resolves a (non-impactful) RuntimeError emitted if a generator is
collected without having been consumed

cassandra/concurrent.py

@@ -166,11 +166,13 @@ class ConcurrentExecutorGenResults(_ConcurrentExecutor):
                     self._condition.wait()
                 while self._results_queue and self._results_queue[0][0] == self._current:
                     _, res = heappop(self._results_queue)
-                    self._condition.release()
-                    if self._fail_fast and not res[0]:
-                        self._raise(res[1])
-                    yield res
-                    self._condition.acquire()
+                    try:
+                        self._condition.release()
+                        if self._fail_fast and not res[0]:
+                            self._raise(res[1])
+                        yield res
+                    finally:
+                        self._condition.acquire()
                     self._current += 1