Update robotstxt.py
Update robotstxt.py

Add message to IgnoreRequest exception so that it can be detectedin the errbak method of a spider
eLRuLL/scrapy,kmike/scrapy,wujuguang/scrapy,finfish/scrapy,ArturGaspar/scrapy,ssteo/scrapy,ssteo/scrapy,finfish/scrapy,kmike/scrapy,kmike/scrapy,wujuguang/scrapy,elacuesta/scrapy,ArturGaspar/scrapy,eLRuLL/scrapy,pawelmhm/scrapy,ArturGaspar/scrapy,scrapy/scrapy,pawelmhm/scrapy,elacuesta/scrapy,pablohoffman/scrapy,dangra/scrapy,starrify/scrapy,pablohoffman/scrapy,dangra/scrapy,pawelmhm/scrapy,Ryezhang/scrapy,eLRuLL/scrapy,pablohoffman/scrapy,finfish/scrapy,scrapy/scrapy,Ryezhang/scrapy,elacuesta/scrapy,dangra/scrapy,Ryezhang/scrapy,ssteo/scrapy,starrify/scrapy,wujuguang/scrapy,scrapy/scrapy,starrify/scrapy