Do not throw exceptions when making expectations on a nil subject

This pull request fixes #400, where `NSInternalInconsistencyException` was thrown when trying to add a match verifier when a previously unresolved verifier exists in a given example. In other words, this:

```objc
it(@"should fail", ^{
    [[(id)nil should] equal:@"foo"]; // [FAILED] expected subject not to be nil
    [[(id)nil should] contain:@"bar"]; // [FAILED] expected subject not to be nil
}
```

now fails with two `expected subject not to be nil` failures, but **doesn't throw any exceptions**.