AsyncRLock: don't swallow Lock.acquire errors #944
                
     Merged
            
            
          
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This PR addresses point 2 ("Missing error handling in
_acquire_non_blocking") raised in #937.It's not clear if or when
asyncio.Lock.acquire()would raise an exception. But should it, our wrapping AsyncRLock should not silently swallow it and worse pretend the lock was successfully acquired.