The TAP spec says: "Error documents should be in a format that matches the requested format where possible; see DALI (Dowler and Demleitner et al., 2017) for details." DALI says: "An error document describing errors in use of the DAL service protocol may be a VOTable document or a plain text document." We should be supporting both VOTable and plain text error docs.
Currently, we expect error endpoint of async TAP service to return a VOTable. (See getErrorMsg of AsyncTapQuery.java) If an error doc is a plain text, we display