[CT-1993] Raise an exception from ref if referencing a private model across groups

[MichelleArk]

During resolution, raise an exception if a node attempts to ref a private model from a different group than the referenced private model’s group.

Nodes not in a group (group is None) should not be able to reference any private models because they are not part of any group.

Note: exposures can have ref calls, but are not 'groupable'. Following on the logic above, they should not be able to reference any private models because they are not part of any group.

[jtcohen6]

Discussed live: We may want a new class of exception, DependencyError, when:

• ref/source/metric a model/source/metric/etc that doesn't exist
• a model wants to belong to a group that doesn't exist
• ref a model in a different group with access: private

(as distinct from existing CompilationError)

[MichelleArk]

From 2/6 Estimation - ReferenceError would be a preferable name to DependencyError

[jtcohen6]

As @emmyoop rightly knew (or guessed), we already have an exception named DependencyError, and it's only used within dbt deps.

ReferenceError all the way!