Issue warning when kerning against Zzzz (Unknown) script

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".

Search the codebase for the current warning text, “tried to kern Latn against Zzzz, skipping pair,” and inspect the entry point that handles cross-script kerning. Reproduce the case with a PUA glyph and update the warning to explain the unknown script and unreliable shaping behavior, then run the related tests if available.

Written by the indexing model from the issue text.