title: Raised exception seemingly not caught
body: <p>I run an application with <code>futures.ProcessPoolExecutor(max_workers=4)</code>. On linux, it runs fine. On two different macos, it freezes/locks/we don't know. When we hit CTRL+C on the macoss, the following stack trace is printed.</p>
  
  <pre><code>Traceback (most recent call last):
  Traceback (most recent call last):
  Traceback (most recent call last):
  Traceback (most recent call last):
    File "tests/parallel/recorder_tool.py", line 168, in __enter__
    File "tests/parallel/recorder_tool.py", line 168, in __enter__
    File "tests/parallel/recorder_tool.py", line 168, in __enter__
    File "tests/parallel/recorder_tool.py", line 168, in __enter__
      self.manager = self.__class__.managers.pop()
      self.manager = self.__class__.managers.pop()
      self.manager = self.__class__.managers.pop()
  IndexError: pop from empty list
  IndexError: pop from empty list
  
  During handling of the above exception, another exception occurred:
  
      self.manager = self.__class__.managers.pop()
  IndexError: pop from empty list
  
  During handling of the above exception, another exception occurred:
  
  KeyboardInterrupt
  
  During handling of the above exception, another exception occurred:
  
  KeyboardInterrupt
  IndexError: pop from empty list
  KeyboardInterrupt
  Exception ignored in: 'grpc._cython.cygrpc._next'
  ...
  </code></pre>
  
  <p>Here is the code sample related to the <code>IndexError</code>.</p>
  
  <pre><code>try:                                            
      self.manager = self.__class__.managers.pop()
  except IndexError:                              
      self.manager = Factory.get_manager(get_conn)
  </code></pre>
  
  <p>There is an <code>except IndexError</code> block, but the stack trace makes it look like it's never reached. Is there something really fishy that I should investigate here or is it simply because Ctrl+C was pressed so no more exception handler work? Also, is there any obvious reason why it would work on linux and not on macos?</p>
  
  <p>Thanks!</p>