pool: Decrement ._opened_conns when connect() raises exception

[methane]

No description provided.

[eventlistener]

@methane Hi! May I ask why this doesn't get merged?

[methane]

see https://github.com/PyMySQL/Tornado-MySQL#warning

[ycraaron]

I have found the same problem today. I think this is a bug level error and this PR should be merged.
@methane

[ycraaron]

I think this should be merged to master because it's not a new feature instead it's a bug related to max_open_connections. Here is the problem I have related to this pr. After reading the code and figuring out the problem, I find this pr, which I think is a good fix for it, hope this could be merged to master.
AmbiLabs#1

[methane]

I'll merge this this time, but please don't expect I fix more bugs anymore.
I strongly recommend you to Go asyncio.

[ycraaron]

Thanks, bro!
Our problems seem to be more complicated for the time being and Tornado-MySQL is a good choice.
Yeah, we will explore the asyncio, thanks for this merge!