Improve error message when callback is not callable
Improve error message when callback is not callable

Ryezhang/scrapy,Parlin-Galanodel/scrapy,dangra/scrapy,ArturGaspar/scrapy,wujuguang/scrapy,starrify/scrapy,Ryezhang/scrapy,finfish/scrapy,ArturGaspar/scrapy,ssteo/scrapy,pawelmhm/scrapy,ssteo/scrapy,ssteo/scrapy,Ryezhang/scrapy,pablohoffman/scrapy,eLRuLL/scrapy,umrashrf/scrapy,elacuesta/scrapy,eLRuLL/scrapy,pablohoffman/scrapy,starrify/scrapy,scrapy/scrapy,Parlin-Galanodel/scrapy,kmike/scrapy,kmike/scrapy,starrify/scrapy,umrashrf/scrapy,elacuesta/scrapy,pablohoffman/scrapy,Parlin-Galanodel/scrapy,finfish/scrapy,dangra/scrapy,pawelmhm/scrapy,ArturGaspar/scrapy,wujuguang/scrapy,finfish/scrapy,umrashrf/scrapy,elacuesta/scrapy,eLRuLL/scrapy,pawelmhm/scrapy,scrapy/scrapy,wujuguang/scrapy,dangra/scrapy,scrapy/scrapy,kmike/scrapy