Provide better assertion feedback for missing view

If you assert that a view has a variable, but a View instance hasn't yet been returned, then a nasty fatal error will be displayed.

```bash
.PHP Fatal error:  Call to a member function getData() on a non-object
```

This doesn't really help when following a TDD process. This commit simply checks whether the getData method exists. If it doesn't, then it provides more actionable feedback.

```bash
1) ExampleTest::testAssignsOrdersToView
A view has not yet been loaded.
```