redis manager, verbose error logging

[Eugnee]

Sometimes it's very useful to see exact error from redis in logs

[miguelgrinberg]

Can we instead print the exception message? The stack trace here is not very useful. My concern is that for every person who wants more verbosity in the logs there's going to be another that complains that the logs are too verbose after this change is made.

[Eugnee]

I usually prefer full stack trace, but you probably right.
In this case, I guess something like str(exc) should be fine.