Get rid of unchecked type assertions

[jbarrick-mesosphere]

What happened:

The test harness has some unchecked type assertions that cause panics.

=== PAUSE TestHarnessRunIntegration/harness/create-or-update
panic: interface conversion: runtime.Object is *unstructured.Unstructured, not *v1alpha1.TestStep [recovered]
    panic: interface conversion: runtime.Object is *unstructured.Unstructured, not *v1alpha1.TestStep

goroutine 1036 [running]:
testing.tRunner.func1(0xc00051a300)
    /usr/local/Cellar/go/1.12.5/libexec/src/testing/testing.go:830 +0x392
panic(0x20e3ee0, 0xc0001b8cf0)
    /usr/local/Cellar/go/1.12.5/libexec/src/runtime/panic.go:522 +0x1b5
github.com/kudobuilder/kudo/pkg/test.(*Step).LoadYAML(0xc000124be0, 0xc000045180, 0x4e, 0x0, 0x0)
    /Users/gaston/mesosphere/kudo/pkg/test/step.go:373 +0xceb
github.com/kudobuilder/kudo/pkg/test.(*Case).LoadTestSteps(0xc000258230, 0xc000292730, 0x10)
    /Users/gaston/mesosphere/kudo/pkg/test/case.go:177 +0x326
github.com/kudobuilder/kudo/pkg/test.(*Harness).RunTests.func1(0xc00051a300)
    /Users/gaston/mesosphere/kudo/pkg/test/harness.go:252 +0x15e
testing.tRunner(0xc00051a300, 0xc0002fdbf0)
    /usr/local/Cellar/go/1.12.5/libexec/src/testing/testing.go:865 +0xc0
created by testing.(*T).Run
    /usr/local/Cellar/go/1.12.5/libexec/src/testing/testing.go:916 +0x35a
FAIL    github.com/kudobuilder/kudo/pkg/test    10.061s

Ideally we could add a linter that would detect all of these.

[porridge]

In the end I enabled reporting of the unchecked type assertions in #1245 usingerrcheck. It's not perfect but seems to catch at least the cases we had in our codebase.