Expand node-group-template usage validation msg

Now that we have a CLI, the error messages returned from validation
are presented directly to the user. Currently, node group templates
cannot be deleted if they are in use. This change expands the error
message to tell the user where the node group templates are in use.

Change-Id: I3764aa6730db52f2a313626054c941eb3a9723bc
Merge "Expand node-group-template usage validation msg"