Return better error for when trying to start a task with a missing plugin

[geauxvirtual]

Related to #1210, when trying to start a task after a plugin has been removed, the only error returned is plugin not found, not which plugin was not found.

Here, https://github.com/intelsdi-x/snap/blob/master/control/subscription_group.go#L400, we are just returning the error from trying to get the loaded plugin, which is just a simple plugin not found. As opposed to here, https://github.com/intelsdi-x/snap/blob/master/control/subscription_group.go#L258, where the error is rewritten to include the plugin that could not be found.