Regrettably, upgrade from 4.5.3 to 5.6.2 woes not finished - ModuleLoadException no longer handled gracefully as in 4.5.3...

Long story short, we recently inherited a DNN site which was developed by another firm some time ago.  After much pain during the upgrade (following upgrade path), and after overcoming many obstacles, the site is practically 100% sound except for a ModuleLoadException no longer being handled gracefully by version 5.6.2 - whereas version 4.5.3 cleanly displayed the error with a nice red exclamation mark along side it without blowing up the page.

Not sure why the site attempts to use a module that no longer exists, but that's almost beside the point.  The question here is why is the most recent version of DNN blowing up with this exception when the older version does not?  Furthermore, what is the recommended course of action?

One would ask why would a site Admin try to use a module that was obviously never used before?  But, again the point here is why is DNN no longer handling the exception cleanly?

Appreciate any advice or help.