Infinite loop of rendering errors in FastBoot

So this is a bit of tricky one, and I'm not sure if it's this add or `ember-page-title` causing it.

Basically, under a _very specific_ set of circumstances, a rendering error (i.e. trying to invoke a non-existent helper) will trigger an infinite loop of errors in FastBoot that ends up spiking the entire fastboot-app-server.

I've put together a [minimal reproduction](https://github.com/davewasmer/fastboot-error-handling). Clone the repo down, `ember s`, and hit `localhost:4200` to see it in action. As a heads up - once it starts dumping the error in a loop, it stops responding to SIGTERMs (Ctrl-C), so you'll need to forcibly kill the process (i.e. "Force Quit" on Mac or SIGKILL with `kill -9`).

I'm at a loss as to what might cause this - the errors being dumped aren't super helpful. After the initial error about being unable to find the helper, it just starts dumping this repeatededly:

```
TypeError: Cannot read property 'syscall' of null
at AppendOpcodes.evaluate (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/@glimmer/runtime.js:69:1)
at LowLevelVM.evaluateSyscall (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/@glimmer/runtime.js:3269:1)
at LowLevelVM.evaluateInner (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/@glimmer/runtime.js:3215:1)
at LowLevelVM.evaluateOuter (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/@glimmer/runtime.js:3207:1)
at VM.next (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/@glimmer/runtime.js:5262:1)
at TemplateIteratorImpl.next (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/@glimmer/runtime.js:5298:1)
at RootState.render (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/@ember/-internals/glimmer.js:5883:1)
at TransactionRunner.runInTransaction (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/@ember/-internals/metal.js:593:1)
at runInTransaction (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/@ember/-internals/metal.js:728:1)
at InertRenderer._renderRoots (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/@ember/-internals/glimmer.js:6179:1)
at InertRenderer._renderRootsTransaction (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/@ember/-internals/glimmer.js:6216:1)
at InertRenderer._revalidate (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/@ember/-internals/glimmer.js:6262:1)
at invokeWithOnError (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/backburner.js:344:1)
at Queue.flush (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/backburner.js:226:1)
at DeferredActionQueues.flush (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/backburner.js:423:1)
at Backburner._end (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/backburner.js:957:1)
at Backburner.end (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/backburner.js:707:1)
at Backburner._run (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/backburner.js:1006:1)
at Backburner._join (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/backburner.js:986:1)
at Backburner.join (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/backburner.js:757:1)
at Array.loopEnd (/var/folders/p4/2wqjhkg160sfdr7pykp6f3ch0000gn/T/broccoli-72882ZYE8RoqN9B1E/out-221-broccoli_merge_trees/assets/@ember/-internals/glimmer.js:6008:1)
[...snip ~100 more lines of repetitive backburner frames...]
```

Some observations I've made (not sure of the relevance). The problem only seems to occur if:

* **the non-existent helper is invoked in a child template.** If I move the invocation from `index.hbs` into `application.hbs`, it's fine.
* **both `{{head-layout}}` and `{{title}}` invocations are present, and they both occur before the `{{outlet}}` invocation.** Remove either, or move either below the `{{outlet}}`, and it's fine.

Things that don't seem to matter:
* **the order of invocation for `{{head-layout}}` and `{{title}}`**. Either one first, still hit the problem.
* **the contents of `head.hbs`**. I can comment out the entire file and it still occurs.