I think I understand the goal here—you're worried about not being able to load assemblies for some reason, and now that users can specify assembly names explicitly, you want to provide them with helpful messages when we can't.
I'm concerned, though, that this change will result in a lot of error messages whenever an unmanaged assembly is specified in the list of extra assemblies (or is in the working directory, if we're using the DefaultBootstrapper). I think we should avoid complaining about unmanaged assemblies if at all possible, to avoid alarming people for no good reason, especially given that those assemblies couldn't possible contain extensions anyhow.