Add response error constant to Redis managers so that exceptions can be caught sanely.

[hodgestar]

Currently one has to do:

try:
    redis.op(...)
except Exception:
    ...

for most redis operations because the class of exception raised depends on the kind of redis one has (sync, async, or fake).

Adding a RESPONSE_ERROR attribute to the redis manager would allow us to do:

try:
    redis.op(...)
except redis.RESPONSE_ERROR:
    ...

Much nicer.

[hodgestar]

@KaitCrawford Ready for review.

[KaitCrawford]

👍 looks good to me