improve error message for unix socket bind failure

[neomantra]

Currently when a unix socket open fails, redis-server outputs:
# Opening socket: bind: Permission denied

Since the code path for that is exclusively for unix socket failures (https://github.com/antirez/redis/blob/unstable/src/redis.c#L1784), I suggest the error message prefix be changed to:
# Opening unix socket:

The motivation is to make operations easier. I was receiving this error and started exploring networking aspects, rather than the incorrect permissions on the unix socket file. A slightly more verbose message would have immediately narrowed the space of possible failure modes.

The downside of this change is that it would break exisitng ops/logging infrastructure that looks explicitly for the existing message prefix.

[antirez]

Thanks, fixed. I don't think we can afford to consider error logs as part of our API so while to break it without a good reason is probably not worth it, to fix errors is a good enough reason to potentially break regexp-grepping. However I'll avoid to merge in other branches if not unstable to allow enough time to notice the change. Cheers.