Handle invalid subject event when subject contains no nodes in a particular year

[AnimeAllstar]

If we generate the subject graph for STAT and limit courses to year 1 (https://www.ubsee.dev/subject/STAT?year=1), we get an Invalid Subject error because no year 1 stat courses exist. This should be handled differently. Preferable another error type like 'No courses for query: STAT, year 1' or something like that.