I applied this to an elint error message.  I was a bit confused by
"Unable to find require'd library" until realizing it wanted the source,
where I only had the .elc.  I think I struck other errors from some of
my macro expansions or something which I can't reproduce any more, but
using error-message-string lets it show whatever went wrong.

You'd be tempted to make it a display-warning or in the elint report
buffer, instead of just a `message', so as not to lose it among other
chattering.  But that can be for another time ...