Add BusyLoadingError exception.

[yossigo]

Currently -LOADING errors are handled as connection errors and the client has no way to differentiate.

This introduces BusyLoadingError to differentiate this condition.

[andymccurdy]

I'm not certain we can do this without further overhaul. LOADING currently raises a ConnectionError so that it forces a disconnection. This is needed when connecting to a Redis server that requires a password or to a database other than 0. The AUTH and SELECT commands are issued from the Connection's on_connect method which only get run when the socket is connected.

[yossigo]

That's my mistake, BusyLoadingError should inherit ConnectionError so that it gets handled the same way and no additional changes will be required, but the client will still be able to tell the difference.

[andymccurdy]

Sounds good. Thanks!

[LK4D4]

It seems that sometimes we can get from command execution exception instances.
Look at redis/connection.py:100

return self.EXCEPTION_CLASSES[error_code](response)

but in redis/connection.py:306

if isinstance(response, ResponseError): 
    raise response

we catch only ResponseError, so all ConnectionError(or derived exceptions) instances go direct to client as answer from command. So, we need to process ConnectionError here too.

[andymccurdy]

@LK4D4 thanks for tracking this down. Fix is in, I'll make a 2.7.6 release now so these fixes are on PyPI.