cellml
Make sure all Issues have a meaningful reference
Throughout the code we should take advantage of having levels and specification categories for Issues so that they can be more useful to the user. Applies to the Generator class, but also throughout.
Now that we have levels of issues, it would be good to test for things like blank strings passed to the generator profile where something "useful" is a non-blank string? This means also that it would be good to set the UNSPECIFIED reference rule to something more (ahem) useful ;) so that we can also tie it to proper documentation and user help.
Hmm, maybe, but this is beyond the scope of this PR. Feel free to create a new issue for it, if you want.
Originally posted by @agarny in https://github.com/cellml/libcellml/issues/650#issuecomment-736950091
