typechecker+tests: Use the most specific known scope id for unknown namespaces

[cg-jl]

unwrapping isn't an option if one of the namespaces in the access chain isn't found. Since checked_namespaces is emmited even when the namespace wasn't found (so that typecheck/IDE can continue knowing there was a namespace access), using the last scope id from the last known namespace when the next one wasn't found.

Fixes #560.