incr/decr returns 0 instead of None when can't connect to server

[Ivan Virabyan]

Consider this code:

hits = cache.incr('hits')
if hits is None:
    hits = 1
    cache.set('hits', hits)
assert hits != 0

Variable 'hits' here must always be a positive integer. This works fine, but in vary rare cases variable 'hits' gets 0 value. I looked at the code, and found out, that incr/decr returns zero when it wasn't able to connect to the memcached:
        server, key = self._get_server(key)
        if not server:
            return 0

But when key doesn't exist, or there was a socket error while sending the command, return value is None.
This is very confusing. I can't distingish error result, from success increment of -1.
I think it'd be more appropriate to return None when there were connection errors.

[Sean Reifschneider (jafo)]

Updated _incrdecr code and documentation and committed to github. Thanks!