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
For whatever reason, the existing fixtures were set up bound to a context rather than assigned as `describe`-local variables.

As it stands, the `bind` is necessary here as `this.myConcreteRegion` will resolve against the global object when the expectation is evaluated.

```
1) region when creating a new region and the "el" does not exist in DOM when showing a view should throw an exception saying an "el" doesnt exist in DOM:

    AssertionError: expected [Function] to throw error including 'An "el" #not-existed-region must exist in DOM' but got 'Cannot read property \'myConcreteRegion\' of undefined'
    + expected - actual

    +An "el" #not-existed-region must exist in DOM
    -Cannot read property 'myConcreteRegion' of undefined
    
2) region when creating a new region and the "el" does not exist in DOM when showing a view should not throw an exception:
   AssertionError: expected [Function] to not throw an error but 'TypeError: Cannot read property \'myEphemeralRegion\' of undefined' was thrown
```