ignore unknown packages in .meteor/versions

Hopefully this isn't ignoring real error cases. The whole
previousSolution data-rewriting hack probably needs to be fixed
anyway. But this seems to work?
minor improvement
tiny refactor
Sort of better errors for nonexistent packages

Specifically, it's less good about saying "package doesn't exist" but
does at least include which top level dependency caused the problem
different way to show usage
various minor error cleanups