Print the name of the layer where the incompatible glyphs are?

[thundernixon]

As described in issue #182, I ended up being quite confused by a failure of cu2qu to make glyphs compatible, when I knew their foreground layers were compatible. I eventually found that a support layer, used for other characters in the designspace, had empty versions of these failing glyphs, and this was blocking the conversion.

@anthrotype suggested the cu2qu might be able to print the name of the layer where the incompatible glyphs are. As a user, I expect this could really be a helpful addition! Just leaving this issue here to document the idea.