After some more experience, going through this several more times, I seem to have found a pattern.
Apparently, this kind of problem arises when I reboot the MangoGT without explicitly stopping Mango first.
Having examined the output from systemctl show mango.service -p TimeoutStopUSec which shows TimeoutStopUSec=1min 30s it appears the service file is correctly configured; however, my experience suggests there is a failure somewhere in there. Additionally, a reboot command seems to execute immediately, too soon to properly shut down.
Thanks, Craig, for the info. I've removed the Font Awesome JS reference and can still see the icons, so that's good.
The bad news is that I still get the error posted in my initial post. Not sure what's causing it now. The only thing I thought each page with the error had in common was using Font Awesome, but it must be something else. I even removed all references to Font Awesome and still got an error.
I have a work-around, so it's not the end of the world, just a little annoying.