Paste the code from around line 43 in that file here.
discuss.answer
It's not finding the action with those fields (namespace and controller) in the modx_actions table.
It kind of looks like your files may be for a different version of MODX than that of your database. Is there anything in your modx_actions table?
Thank you, BobRay. turns out you were right. Since my attempt to upgrade had crashed the instant I hit the install button, I assumed the db hadn't been affected and never bothered restoring my original db backup when I restored my folder tree backup. Obviously, that was a dumb assumption. I restored both and everything is back to normal. And the original problem is fixed now too, since I got the http plugin installed.
Your help is very much appreciated. You and everyone who commented with tips has my overwhelming thanks.
I'm glad you got it sorted.