Move validation into assembly resolve

This pull request moves the validation into the resolve event because I ran into a problem where the built in resolution doesn't work for a lot of scenarios like within a VSIX package where the base directory does not have the wrapper assemblies. I wrote my own resolver method and attached before the AssemblyResolver.Initialize(), but didn't matter since the static constructor would throw the exception immediately.

With this change, the intended effect is the same in that it throws an exception if the files aren't there, but it's done lazily allowing other people to do the resolution themselves which prevents this code from ever running.