continuously attempt to start sensu-client

I have also run into https://github.com/sensu/sensu/issues/680.  For me, this happened across one of our production clusters and left all of the sensu-client processes crashed.  As server had not heard from the clients, all clients were marked CRITICAL due to keepalive failures and pages were firing unnecessarily.

This change wraps the start method with a begin/rescue.  If we encounter failure, we will error log it and then retry startup in
```ruby
@settings[:client][:restart_wait]
```
 seconds.  This defaults to 10 seconds which should be plenty to keep both the service and cpu happy.  Once we succeed, indicate so with a log message.

I have yet to add any tests. I wanted to submit this PR for feedback on the approach.