More descriptive error message (and minor test project change)

Here is a pull request for issue #2 that just adds a little more helpful text to the exception in the case that a marker class is defined, but is not defined in the global namespace.

I also noticed that the test project was for .NET 4.5, but the main project was .NET 4.0. Since I'm working on a Windows 7 machine, I changed the test project so that I could run the tests. Is there a reason that the test project is .NET 4.5 that I'm missing?