error codes with per-module prefix.
* src/autofit/afpic.c: Include `aferrors.h'.
* src/cff/cffpic.c: Include `cfferrs.h'.
* src/pshinter/pshpic.c: Include `pshnterr.h'.
* src/raster/rastpic.c: Include `rasterrs.h'.
* src/sfnt/sfntpic.c: Include `sferrors.h'.
* src/smooth/ftspic.c: Include `ftsmerrs.h'.
* src/truetype/ttpic.c: Include `tterrors.h'.