Updated image reader so that it would throw an exception rather then returning a 0 pointer.
Updated image reader so that it would throw an exception rather then returning a 0 pointer.

tomhughes/mapnik,pnorman/mapnik,tomhughes/python-mapnik,davenquinn/python-mapnik,yohanboniface/python-mapnik,garnertb/python-mapnik,Uli1/mapnik,davenquinn/python-mapnik,mbrukman/mapnik,whuaegeanse/mapnik,naturalatlas/mapnik,mapnik/mapnik,lightmare/mapnik,mbrukman/mapnik,whuaegeanse/mapnik,stefanklug/mapnik,mapycz/mapnik,pnorman/mapnik,Airphrame/mapnik,sebastic/python-mapnik,jwomeara/mapnik,pramsey/mapnik,sebastic/python-mapnik,yohanboniface/python-mapnik,mapycz/python-mapnik,cjmayo/mapnik,Uli1/mapnik,zerebubuth/mapnik,stefanklug/mapnik,Uli1/mapnik,jwomeara/mapnik,jwomeara/mapnik,manz/python-mapnik,garnertb/python-mapnik,Airphrame/mapnik,tomhughes/python-mapnik,cjmayo/mapnik,tomhughes/mapnik,mapnik/mapnik,mapnik/mapnik,tomhughes/mapnik,garnertb/python-mapnik,CartoDB/mapnik,stefanklug/mapnik,zerebubuth/mapnik,mapnik/python-mapnik,sebastic/python-mapnik,Airphrame/mapnik,lightmare/mapnik,mbrukman/mapnik,pramsey/mapnik,Uli1/mapnik,mapnik/mapnik,mapycz/mapnik,tomhughes/mapnik,naturalatlas/mapnik,pramsey/mapnik,manz/python-mapnik,mbrukman/mapnik,mapycz/mapnik,pramsey/mapnik,manz/python-mapnik,jwomeara/mapnik,naturalatlas/mapnik,mapnik/python-mapnik,pnorman/mapnik,pnorman/mapnik,CartoDB/mapnik,tomhughes/python-mapnik,davenquinn/python-mapnik,lightmare/mapnik,cjmayo/mapnik,cjmayo/mapnik,whuaegeanse/mapnik,whuaegeanse/mapnik,mapnik/python-mapnik,mapycz/python-mapnik,yohanboniface/python-mapnik,zerebubuth/mapnik,naturalatlas/mapnik,Airphrame/mapnik,stefanklug/mapnik,lightmare/mapnik,CartoDB/mapnik