Log connection failure

Problem/Motivation

When the module could not connect to the memcached server, everything is slow but nothing really failed, no erreor is thown or logged.
This lack of information doesn't help to troubleshoot the site failure.

The info is only visible on an admin page if you enable "Memcache Admin".

Steps to reproduce

1. Setup a Drupal with this module
2. Configure it to use a single memcached server.
3. Test everything is ok
4. Set a dummy IP in $settings['memcache']['servers']
5. Check that the web site is still up but slow and with no error message
6. Check for error messages into the log

Proposed resolution

Add an fatal level log when Drupal could not connect to a memcached server. Use a memory flag so the error should be send only once or at a slow rate (every minute?).