Remove redundant "ERR" from ResponseError message

A tiny patch, but it seems like the original intent was to remove "ERR" from the ResponseError string since this is redundant. Removing ERR in the message is also done in redis-py. The current code is at least peculiar enough to suggest this:

ResponseError(data if data[:4] == 'ERR ' or data) # ??? of course, always data no matter what