Ugly fix for uncaught exceptions in Zabbix code. Needs more work.

At the moment some exceptions are not caught in the Zabbix plugin. On the other end there is a ZabbixError exception defined which nothing raises AFAICT. I guess the idea would be to catch all ZabbixAPIExceptions and turn them into ZabboxErrors?

I'm afraid this requires more time and skill than I have at the moment. Nevertheless, here is a quick and dirty patch which fixes a few real-life crashes (encountered with an unreliable network connection, where the Zabbix API requests can and do time out). I know ths patch is dirty and still needs a lot of work before merging, but it works for me.