This helps to clarify the situations under which these errors can occur, and
the generality also helps to show why the inner types are as they
are (e.g. use of `String`).
But more importantly, this allows for an error type in `finalize` that is
detached from the `ParseState`, which will be able to be utilized in the
lowering pipeline as a more general error distinguishable from other
lowering errors. At the moment I'm maintaining BC, but a following commit
will demonstrate the use case to introduce recoverable vs. non-recoverable
errors.
DEV-13158