Add warning message when plugin isn't configured.

[ghost]

When one applies liquibase plugin, not configures it, and runs "update" task - the console output displays a message "BUILD SUCCESSFUL".
It could be great if plugin displayed a warning that "plugin has not been configured" instead.

[lared]

+1

[stevesaliman]

With release 2.1.0, the plugin now throws an exception when there are no activities defined, which happens when the plugin isn't configured.

Better late than never I suppose :-)