RedisSessionInterface: Flask context not popped on Redis issues

[kilianm]

Redis exceptions on open_session make Flask crash painfully, not popping the context correctly (leaking data across requests).
Flask doesn't catch any possible exceptions on the session_interface, resulting in messing up the context.

To reproduce

1. Set some parameter in flask.g
2. Initiate RedisSessionInterface on an invalid redis server (redis.exceptions.ConnectionError or any other, we had sometimes Timeout exceptions).
3. The next requests contains the old flask.g - context not popped

Fix
Ideally Flask uses its interfaces safely - always popping context. Bug created: pallets/flask#2517
Still, I think it would be nice that Flask-Session is safe as well, catching possible redis problems and not letting Flask deal with the exceptions.

Environment
Python version: 2.7.6
Flask version: 0.10.1
Werkzeug version: 0.11.11
Flask-Session: 0.3.1

[Lxstr]

The heart of this issue is fixed in flask pallets/flask#2254.

Regarding handling in flask-session, I think it is better to use SESSION_REDIS = Redis(host='localhost', port=6379, retry=retry, retry_on_error=[BusyLoadingError, ConnectionError, TimeoutError])