Cross Domain Error on Localhost on Any Error in Google Apps Script

I noticed an odd-feeling error while playing with a frontend AJAX call from a localhost server that fetched data from a Google Sheet via a Google Apps Script middleware script.

Any time there was any kind of error in the Google Apps Script code. The JSON-P-based front end call was receiving this message (using the Chrome dev tools Console):

Cross-Origin Read Blocking (CORB) blocked cross-origin response with MIME type text/html. See for more details.</code>

Technically, of course, it was a showing up as a CORS (Cross Domain Resource Sharing) or CORB (Cross Domain Read Blocking) error.

I bet the Google Apps Script error message automatically sends data back as HTML/Text and this is messing with the JSON-P callback, which is expecting the data type to be “javascript”.