Issue warning when kerning against Zzzz (Unknown) script

[madig]

A glyph with a PUA is determined to be an "Unknown" script, and OpenType forbids cross-script shaping. Khaled noted that this is implementation defined, it may work in Firefox but not Chrome, and a quick spot check confirms this.

Currently, the warning says something like "tried to kern Latn against Zzzz, skipping pair", so a more helpful message would be "tried to kern Latn against an unknown script (glyphs have PUA), this may not work reliably, therefore we drop this pair".