Allow loader failures to 500
Allow loader failures to 500

If a loader error happens and it's not a 404, a 502 nor a 504, currently an AttributeError will happen on that condition, because engine_error doesn't exist for a FetchResult in the loader case.
voxmedia/thumbor,thumbor/thumbor,okor/thumbor,scorphus/thumbor,scorphus/thumbor,marcelometal/thumbor,scorphus/thumbor,marcelometal/thumbor,gi11es/thumbor,thumbor/thumbor,voxmedia/thumbor,kkopachev/thumbor,gi11es/thumbor,voxmedia/thumbor,marcelometal/thumbor,okor/thumbor,kkopachev/thumbor,gi11es/thumbor,thumbor/thumbor,scorphus/thumbor,kkopachev/thumbor,okor/thumbor,thumbor/thumbor,kkopachev/thumbor