Log memcached connection issues, handle case of no healthy servers

Attempts to fix https://github.com/3rd-Eden/node-memcached/issues/16

From what I could tell, the case when it's not possible to connect to any memcached servers is not handled, and this type of issue is not logged.  This pull request adds a handler for the error event on the stream, which calls connectionIssue.  Additionally, if no healthy servers are available, and error will be returned.  If the client implementation comes across this error, it can be handled.

@3rd-Eden is this on the right track?