Log meaningful warning instead of silently dropping invalid substitutions

When trying to Generate TTF from a font where some rlig substitutions
refer to nonexistent glyphnames, the lookup subtable is silently marked
as unused and then dropped. Traced this in GDB and fixed it.
warning: suggest parentheses around '&&' within '||'
Merge pull request #1041 from JoesCat/master

[PATCH] Log a meaningful warning instead of silently dropping invalid substitutions