Error in one of th JSON configuration file should not prevent the startup of JMXTrans

This minor change was implemented after our JMXTrans instance stopped starting when one of the JSON configuration file was invalid. We generate our JMXTrans configuration from puppet and have a few 100's. of them. In my opinion, one invalid configuration file should not prevent the monitoring of all of the other services.

Feel free to ignore this patch your opinion on the matter differs.