When the BookwormAPI fails, it doesn't send an HTML error code for clients to interpret.
Since the most recent commit, I can't find any failed queries from BookwormGUI, so this is low priority, I suppose. Here's a simulated query with an error, which provides a 200 Status Code:
http://bookworm.htrc.illinois.edu/cgi-bin/dbbindings.py?query={%22groups%22:[%22date_year%22],%22counttype%22:%22WordsPerMillion%22,%22words_collation%22:%22Case_Sensitive%22,%22database%22:%22hathipd%22,%22search_limits%22:[{%22word%22:[%22\\\%22],%22date_year%22:{%22$gte%22:1750,%22$lte%22:1923}}],%22method%22:%22return_json%22}