memcached: better error messaging

memcached: better error messaging

MemcachedClient output a generic error message: "Error parsing memcached
response\n" whenever it is not able to read from the socket. It is also
lacking the remote peer it is reading form.

This patch add a new message when fgets( <socket> ) return false and
attempt to get the remote peer address / port to append to the error
message. Might help us find out which memcached server is wild.

Change-Id: If918e824970aaa8231078e42fd28d31e8dd4e319