Reveal problem in SingleTypeKernelAssertions

Update test assertions to reveal a problem in SingleTypeKernelAssertions.BuildFailureMessage()
Two tests will now fail with a System.NullReferenceException
Prevent NullReferenceException

Prevent NullReferenceException when kernel.GetSingleInstanceActivationError(type) does not return an error.
Merge pull request #4 from duncanworthy/master

NullReferenceException in SingleTypeKernelAssertions