Prevent incorrect namespace from breaking your REPL

Fixes #849 

Thanks to the Roslyn APIs, this requires reflection to modify an immutable, private field :)
The problem does not appear on Mono due to different namespace handling there, however once Mono error reporting is improved (there is a separate issue for that) we will have to handle that there too.
This is a real nitpick, but since we're returning `IEnumerable<string>` you can do `return _invalidNamespaces.Select(_ => _);` to avoid redundant full enumerations when people call `Any()`.