Only parse valid models

[mduncans]

Model::parse() will parse models that fail nmtran check. Improve parser to error on models that aren't valid.

[Keats]

See https://github.com/A2-ai/pharos/pull/63/files#diff-0bacf5169979b37255b84b41c155340fa066fc947cda4abdbd0d28545646be55R877-R882 for reference to something that is invalid but we don't recognise it