Fix plugins commands

Change platforms property of IPlugin from string[] to Server.DevicePlatform[] type. Add try-catch when fetching data for specific plugin - this way if we fail while fetching data for one of the plugins, information about rest of them will be shown to user.

Change url to MarketPlace to use https instead of http.

Fixes http://teampulse.telerik.com/view#item/280934
http://teampulse.telerik.com/view#item/280394
Maybe we should print something about this error, so that the user can recover from it.