Improvement: Continue to work if one of the zabbix server is not responding

[Dainii]

Zabbix Vue version: 2.1.1
Browser: Firefox

Issue

When one of the configured Zabbix server is not responding, the plugin doesn't display anything. The little widget icon is grey and on click it only displays the error message: Check server configuration in extension options: Error communicating with <server_address>.

Expected behavior
It could be great that when one or more Zabbix server are not responding, the plugin still display the information of the others.

I didn't try to look at the code, but if you give me some pointers I can try to propose a PR.