use write_error instead of get_error_html
use write_error instead of get_error_html

due to changes in tornado 4 for custom error html

roxyboy/nbviewer,Sage-Bionetworks/nbviewer,bollwyvl/nbviewer,Sage-Bionetworks/nbviewer,thomasyu888/nbviewer,iamjakob/nbviewer,AlfiyaZi/nbviewer,imsparsh/nbviewer,iamjakob/nbviewer,christophelec/nbviewer,roxyboy/nbviewer,Jay-Oh-eN/nbviewer,christophelec/nbviewer,bollwyvl/nbviewer,AlfiyaZi/nbviewer,thomasyu888/nbviewer,Jay-Oh-eN/nbviewer,imsparsh/nbviewer