ConnectionError not handled correctly if there are no connections in the pool

[FranGM]

Hi there!

Recently we ran into an issue where it seems like we were not handling a ConnectionError correctly and tried to call connection.disconnect() (see here ) when connection had a None value, which results in an unhandled exception being raised.

Seems like if you're using ClusterBlockingConnectionPool, and time out before a connection is available a ConnectionError will get raised, which gets mishandled in this case. The rest of the error handling seems correct to me, so I'm going to submit a PR that just checks that connection is not None before attempting to disconnect, and I'll be happy to add any other stuff that you think might be missing.

[FranGM]

I don't know if I can reopen issues but there's another instance of this issue with a fix here: #472