Check for content refs in Canvas config on site:export

Problem/Motivation

Twice now I have unwittingly included media entities in a Canvas config item (once a pattern and once in the footer) and been tripped up by the error when trying to reinstall.

Can we add a check to the site:export command to either throw an error (my preference), or at least a warning to alert the user that they have a problem? The reason I favor an error is there are already lots of warnings about stuff that doesn't really matter, so it's likely to be ignored.