Misleading error when you fail to return a step from makeWrapPlansPlugin
### Description

Misleading error when you fail to return a step from makeWrapPlansPlugin.

### Steps to reproduce

Wrap a plan with a plan that returns no step; e.g. https://github.com/benjie/ouch-my-finger/pull/10/files

### Expected results

Error about forgetting to return a step.

### Actual results

```
Now is not a valid time to call `currentLayerPlan`. This error typically occurs when you attempt to call a Grafast step function from outside of the planning lifecycle - it's important to note that Grafast plans must be resolved synchronously, so check for 'async' or 'setTimeout' or any location where a step function is called outside of a plan resolver. For more information, read about plan resolvers [...]
at t.currentLayerPlan (node_modules/grafast/dist/index.js:445:17417)
at new BaseStep (node_modules/grafast/dist/index.js:445:74046)
at new ExecutableStep (node_modules/grafast/dist/index.js:445:74546)
at new ConstantStep (node_modules/grafast/dist/index.js:470:8242)
at t.constant (node_modules/grafast/dist/index.js:470:9080)
at OperationPlan.planField (node_modules/grafast/dist/index.js:227:20442)
at OperationPlan.processGroupedFieldSet (node_modules/grafast/dist/index.js:227:14525)
at OperationPlan.planSelectionSet (node_modules/grafast/dist/index.js:227:15971)
```

### Additional context

Reported by @purge.

### Possible Solution

Add an assertion; it should instead tell you that you must return a step from your plan.
