Warn about master glyphset/Unicode incompatiblities

Here's a fun fact I've just discovered: in a designspace -> variable font workflow, what happens if the master UFOs have different glyphsets or different Unicode codepoint assignments?

The answer seems to be that the first master file mentioned in the designspace file wins: it decides the glyphset and the Unicode codepoints. Then all the glyphs are collected across the masters *by name*, even if their codepoint assignments are different. Glyphs not in the first master's glyphset are ignored.

I'm not suggesting there's a better way to do this because with UFO -> variable font you are essentially trying to merge a bunch of independent font files. But fontmake provides *no warnings at all* that anything is unusual. We check for contour incompatibility, but nothing else.

(I don't know if this should be considered a fontmake problem or a ufo2ft problem or a fontTools.varLib.merge problem...)