Unhelpful error if you forget to add layers

[alexhroom]

If you forget to add layers to your project, when you try to add the contrast you get the unhelpful error:

Error using contrastsClass/validateContrastModel (line 249)
Only one model value is allowed for custom models

rather than an error telling you that your layers are not defined.

To reproduce: try to run the standardLayersDSPCScript with line 44 (problem.addLayerGroup(Layers)) removed.