I got this working for reasons I don't really understand.
Since the code in main.lua that was displaying the two error message toasts I was getting were in lines 390 and 603, I commented the original code out of those lines and replaced it with your suggested
Code:
text = "Response code: "..code, })
Then, when I asked the Wallabag plugin to download articles, it did. Strange since I was only editing the messages that would pop up in the events triggered for those two lines, not the processing of the events themselves. But now, for the first time, I have 30 articles in my /Wallabag directory.