improve error message
improve error message

fabianrost84/cython,JelleZijlstra/cython,cython/cython,slonik-az/cython,slonik-az/cython,hickford/cython,hickford/cython,roxyboy/cython,slonik-az/cython,andreasvc/cython,fperez/cython,andreasvc/cython,fperez/cython,andreasvc/cython,achernet/cython,fabianrost84/cython,hhsprings/cython,dahebolangkuan/cython,mrGeen/cython,madjar/cython,hickford/cython,fperez/cython,mrGeen/cython,JelleZijlstra/cython,fabianrost84/cython,fabianrost84/cython,slonik-az/cython,ChristopherHogan/cython,scoder/cython,cython/cython,andreasvc/cython,madjar/cython,hhsprings/cython,c-blake/cython,fperez/cython,achernet/cython,hhsprings/cython,roxyboy/cython,encukou/cython,mcanthony/cython,c-blake/cython,JelleZijlstra/cython,acrispin/cython,hickford/cython,cython/cython,dahebolangkuan/cython,mcanthony/cython,slonik-az/cython,mcanthony/cython,madjar/cython,marscher/cython,mcanthony/cython,da-woods/cython,marscher/cython,cython/cython,andreasvc/cython,acrispin/cython,mrGeen/cython,encukou/cython,JelleZijlstra/cython,acrispin/cython,scoder/cython,mcanthony/cython,achernet/cython,mrGeen/cython,marscher/cython,da-woods/cython,acrispin/cython,madjar/cython,roxyboy/cython,hickford/cython,c-blake/cython,hhsprings/cython,marscher/cython,roxyboy/cython,fabianrost84/cython,c-blake/cython,encukou/cython,marscher/cython,scoder/cython,encukou/cython,acrispin/cython,c-blake/cython,hhsprings/cython,ChristopherHogan/cython,da-woods/cython,dahebolangkuan/cython,ChristopherHogan/cython,fperez/cython,scoder/cython,dahebolangkuan/cython,da-woods/cython,dahebolangkuan/cython,mrGeen/cython,encukou/cython,achernet/cython,achernet/cython,JelleZijlstra/cython,roxyboy/cython,madjar/cython