Unbound variable warning/exception

[Christopher Lam]

Hi guilers, I saw in guix the incredibly useful unbound-variable exception
printer written by Ludovic. I've adapted for use in gnucash as
https://github.com/Gnucash/gnucash/commit/6f951784 -- there are now in
guile 3.0.4 many more unbound-var warnings than in 3.0.2 -- wouldn't it be
a useful hint to add to the warning in core guile as well?