Fix error logger crash

[Technologicat]

Avoid crashing in error logger when the message of a macro expansion failure is not a string.

This happens more often than one would think, when developing new macros :)

[Technologicat]

Ping? This is a straightforward bugfix, so I'd like to get it into the official version of macropy3, if possible. :)

[azazel75]

It would be nice to have a testcase ;-)

[Technologicat]

Now that I noticed the conflicts alert, it seems you already fixed this - in exactly the same way as I did - in 9cac7df. ;)

But I suppose I can provide a test before closing this.