Promote PLTE warnings to Exceptions. Found by brokensuite's plte_length_mod_three.png
Promote PLTE warnings to Exceptions.
Found by brokensuite's plte_length_mod_three.png


git-svn-id: 319eb561774c901116a2c19d0db24e717df3a3eb@166 40d66c6c-0ee5-11de-8837-45bce846067a

drj11/pypng,drj11/pypng,mnaberez/pypng,Scondo/purepng,mnaberez/pypng,Scondo/purepng