Allow non-DOM-backed regions

* Factor error-throwing out of `_ensureElement` to `_throwOnMissingRegionElement`
* Spec overriding `_throwOnMissingRegionElement` to no-op
* `_ensureElement` returns boolean, indicating whether or not element is available
* `Region#show` early-terminates on missing element
I know this is how it was in the file, but while you're in here...

`expect(this.myConcreteRegion.hasView()).to.be.false`