Migrator should error when invalid variables and mixins are encountered

[jathak]

Right now, if an unrecognized member is encountered during the references pass, it's silently ignored. For functions, this is the best we can do, since the unrecognized function could be a CSS function (or part of a post-processor). However, we should have enough context to error when unrecognized variables and mixins are encountered. This should prevent the migrator from improperly migrating a file that it doesn't have sufficient context for.