Raise if the attached namespace is missing during linearization

[vinistock]

Motivation

Linearization for singletons currently fails with a no method error if we have never indexed the attached class.

This happens due to a shortcoming of the current implementation: we only index files once they are saved, rather than indexing them during modifications while computing all of the other features (using the dispatcher).

See the manual tests to understand what I mean.

Implementation

For now, we need to error properly if the attached class does not exist.

Automated Tests

Added a test.

Manual Tests

1. Create a new file
2. Enter the following code (do not save the file)

class Foo
  # cursor here
end

3. Now, in the position of the cursor, add an @ symbol

The moment the @ symbol is added, we need to linearize ancestors for the singleton class of Foo to provide completion for class instance variables. However, because the file has not been saved, we never indexed Foo in the first place.

On main that fails and on this branch we simply don't show completion until the file is saved.