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.
`return _invalidNamespaces.Select(_ => _);` will also give immutability since it will just yield out each member of the `HashSet`, one by one.

Anyway, this is minor. It's not like there will be 100,000 invalid namespaces. Or at least I hope not :wink: