[lltx] Missing glyphs in font

[Yves Scherrer]

  One more little thing:

When I typeset a document with a glyph that is missing in the selected 
font, the XeLaTeX logfile contains a nice warning:

Missing character: There is no ✓ in font Gentium 
Basic/ICU:script=latn;langua
ge=DFLT;!

When I do the same with LuaLaTeX, I get the rather cryptic message:

luaotfload | otf process: missing cache for lookup cs_l_0_s of type 
gsub_contex
t in font 21 (Gentium Basic)

It would be nice to phrase this message in a way that is understandable 
for the common LaTeX user :)

(In addition, I ask myself if this shouldn't be written to the standard 
output, rather than just to the log file. After all, missing characters 
change the content of the file and are thus "worse" than mere layout 
problems like, say, overfull and underfull boxes.)

What do you think?