Improve exception message when a type cannot be converted during marshaling

[aarononeal]

Now when a type cannot be converted a more detailed exception message is generated stating the types converted from and to. This makes it easier to debug applications.

[marek-safar]

This is what --debug=casts options is for

[aarononeal]

I think handling it directly in the code is less of a hassle and makes for a better out of box debugging experience. I understand though if you're fine with devs having to update their projects with parameter options for debugging, so feel free to close this pull if you decide not to incorporate it.