allow user report of some errors with HAXEDEBUG=1 env var (keep different from OCAMLRUNPARAM)

git-svn-id: http://haxe.googlecode.com/svn/trunk@6367 f16182fa-f095-11de-8f43-4547254af6c6
tells which file has issue when failing some 24 bit PNG decoding

git-svn-id: http://haxe.googlecode.com/svn/trunk@6368 f16182fa-f095-11de-8f43-4547254af6c6