set bg_error_ when background flush goes wrong
set bg_error_ when background flush goes wrong

Summary: as title

Test Plan: unit test

Reviewers: haobo, igor, sdong, kailiu, dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15435

tsheasha/rocksdb,bbiao/rocksdb,tschottdorf/rocksdb,lgscofield/rocksdb,Applied-Duality/rocksdb,fengshao0907/rocksdb,wenduo/rocksdb,lgscofield/rocksdb,facebook/rocksdb,alihalabyah/rocksdb,wat-ze-hex/rocksdb,amyvmiwei/rocksdb,msb-at-yahoo/rocksdb,skunkwerks/rocksdb,ryneli/rocksdb,SunguckLee/RocksDB,wskplho/rocksdb,fengshao0907/rocksdb,Andymic/rocksdb,wenduo/rocksdb,vashstorm/rocksdb,jalexanderqed/rocksdb,zhangpng/rocksdb,virtdb/rocksdb,tschottdorf/rocksdb,kaschaeffer/rocksdb,caijieming-baidu/rocksdb,bbiao/rocksdb,IMCG/RcoksDB,anagav/rocksdb,rDSN-Projects/rocksdb.replicated,geraldoandradee/rocksdb,anagav/rocksdb,siddhartharay007/rocksdb,luckywhu/rocksdb,siddhartharay007/rocksdb,tschottdorf/rocksdb,Andymic/rocksdb,kaschaeffer/rocksdb,NickCis/rocksdb,vmx/rocksdb,OverlordQ/rocksdb,facebook/rocksdb,bbiao/rocksdb,msb-at-yahoo/rocksdb,wat-ze-hex/rocksdb,NickCis/rocksdb,mbarbon/rocksdb,caijieming-baidu/rocksdb,caijieming-baidu/rocksdb,ylong/rocksdb,zhangpng/rocksdb,facebook/rocksdb,flabby/rocksdb,facebook/rocksdb,rDSN-Projects/rocksdb.replicated,hobinyoon/rocksdb,tschottdorf/rocksdb,biddyweb/rocksdb,mbarbon/rocksdb,anagav/rocksdb,wlqGit/rocksdb,IMCG/RcoksDB,vmx/rocksdb,RyanTech/rocksdb,wlqGit/rocksdb,vashstorm/rocksdb,sorphi/rocksdb,biddyweb/rocksdb,rDSN-Projects/rocksdb.replicated,lgscofield/rocksdb,wenduo/rocksdb,JoeWoo/rocksdb,amyvmiwei/rocksdb,Andymic/rocksdb,caijieming-baidu/rocksdb,Andymic/rocksdb,zhangpng/rocksdb,IMCG/RcoksDB,msb-at-yahoo/rocksdb,temicai/rocksdb,Applied-Duality/rocksdb,vmx/rocksdb,temicai/rocksdb,hobinyoon/rocksdb,virtdb/rocksdb,skunkwerks/rocksdb,luckywhu/rocksdb,alihalabyah/rocksdb,virtdb/rocksdb,jalexanderqed/rocksdb,amyvmiwei/rocksdb,geraldoandradee/rocksdb,JohnPJenkins/rocksdb,sorphi/rocksdb,IMCG/RcoksDB,siddhartharay007/rocksdb,geraldoandradee/rocksdb,Applied-Duality/rocksdb,Applied-Duality/rocksdb,RyanTech/rocksdb,biddyweb/rocksdb,Vaisman/rocksdb,JohnPJenkins/rocksdb,wat-ze-hex/rocksdb,siddhartharay007/rocksdb,JohnPJenkins/rocksdb,zhangpng/rocksdb,JoeWoo/rocksdb,siddhartharay007/rocksdb,makelivedotnet/rocksdb,makelivedotnet/rocksdb,OverlordQ/rocksdb,hobinyoon/rocksdb,JackLian/rocksdb,NickCis/rocksdb,vmx/rocksdb,sorphi/rocksdb,makelivedotnet/rocksdb,wskplho/rocksdb,temicai/rocksdb,facebook/rocksdb,ylong/rocksdb,JackLian/rocksdb,sorphi/rocksdb,wenduo/rocksdb,flabby/rocksdb,dkorolev/rocksdb,biddyweb/rocksdb,rDSN-Projects/rocksdb.replicated,ylong/rocksdb,caijieming-baidu/rocksdb,amyvmiwei/rocksdb,luckywhu/rocksdb,wskplho/rocksdb,NickCis/rocksdb,NickCis/rocksdb,tizzybec/rocksdb,RyanTech/rocksdb,makelivedotnet/rocksdb,ryneli/rocksdb,jalexanderqed/rocksdb,sorphi/rocksdb,dkorolev/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,norton/rocksdb,flabby/rocksdb,norton/rocksdb,ryneli/rocksdb,anagav/rocksdb,msb-at-yahoo/rocksdb,temicai/rocksdb,vmx/rocksdb,hobinyoon/rocksdb,dkorolev/rocksdb,JohnPJenkins/rocksdb,RyanTech/rocksdb,IMCG/RcoksDB,Vaisman/rocksdb,RyanTech/rocksdb,skunkwerks/rocksdb,vmx/rocksdb,virtdb/rocksdb,JackLian/rocksdb,Vaisman/rocksdb,facebook/rocksdb,geraldoandradee/rocksdb,jalexanderqed/rocksdb,wlqGit/rocksdb,wat-ze-hex/rocksdb,vashstorm/rocksdb,luckywhu/rocksdb,makelivedotnet/rocksdb,bbiao/rocksdb,alihalabyah/rocksdb,vashstorm/rocksdb,flabby/rocksdb,bbiao/rocksdb,ryneli/rocksdb,anagav/rocksdb,hobinyoon/rocksdb,Applied-Duality/rocksdb,temicai/rocksdb,lgscofield/rocksdb,skunkwerks/rocksdb,tizzybec/rocksdb,flabby/rocksdb,bbiao/rocksdb,luckywhu/rocksdb,jalexanderqed/rocksdb,msb-at-yahoo/rocksdb,luckywhu/rocksdb,OverlordQ/rocksdb,fengshao0907/rocksdb,norton/rocksdb,fengshao0907/rocksdb,flabby/rocksdb,rDSN-Projects/rocksdb.replicated,SunguckLee/RocksDB,JohnPJenkins/rocksdb,sorphi/rocksdb,virtdb/rocksdb,norton/rocksdb,facebook/rocksdb,temicai/rocksdb,fengshao0907/rocksdb,dkorolev/rocksdb,facebook/rocksdb,JackLian/rocksdb,bbiao/rocksdb,alihalabyah/rocksdb,caijieming-baidu/rocksdb,wlqGit/rocksdb,tsheasha/rocksdb,skunkwerks/rocksdb,Vaisman/rocksdb,geraldoandradee/rocksdb,wenduo/rocksdb,wat-ze-hex/rocksdb,zhangpng/rocksdb,vashstorm/rocksdb,SunguckLee/RocksDB,RyanTech/rocksdb,rDSN-Projects/rocksdb.replicated,vashstorm/rocksdb,biddyweb/rocksdb,OverlordQ/rocksdb,wlqGit/rocksdb,jalexanderqed/rocksdb,makelivedotnet/rocksdb,kaschaeffer/rocksdb,wenduo/rocksdb,amyvmiwei/rocksdb,Vaisman/rocksdb,NickCis/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,kaschaeffer/rocksdb,lgscofield/rocksdb,Andymic/rocksdb,wenduo/rocksdb,lgscofield/rocksdb,tizzybec/rocksdb,virtdb/rocksdb,dkorolev/rocksdb,JoeWoo/rocksdb,amyvmiwei/rocksdb,JoeWoo/rocksdb,vmx/rocksdb,Vaisman/rocksdb,wskplho/rocksdb,tschottdorf/rocksdb,SunguckLee/RocksDB,anagav/rocksdb,biddyweb/rocksdb,norton/rocksdb,fengshao0907/rocksdb,JoeWoo/rocksdb,IMCG/RcoksDB,tizzybec/rocksdb,siddhartharay007/rocksdb,wskplho/rocksdb,zhangpng/rocksdb,zhangpng/rocksdb,tizzybec/rocksdb,kaschaeffer/rocksdb,rDSN-Projects/rocksdb.replicated,kaschaeffer/rocksdb,mbarbon/rocksdb,IMCG/RcoksDB,msb-at-yahoo/rocksdb,SunguckLee/RocksDB,kaschaeffer/rocksdb,SunguckLee/RocksDB,JackLian/rocksdb,norton/rocksdb,JoeWoo/rocksdb,wenduo/rocksdb,lgscofield/rocksdb,ylong/rocksdb,JoeWoo/rocksdb,wskplho/rocksdb,mbarbon/rocksdb,tsheasha/rocksdb,mbarbon/rocksdb,wskplho/rocksdb,wat-ze-hex/rocksdb,tizzybec/rocksdb,alihalabyah/rocksdb,tsheasha/rocksdb,Applied-Duality/rocksdb,dkorolev/rocksdb,ryneli/rocksdb,norton/rocksdb,geraldoandradee/rocksdb,norton/rocksdb,RyanTech/rocksdb,Andymic/rocksdb,JohnPJenkins/rocksdb,tizzybec/rocksdb,virtdb/rocksdb,Vaisman/rocksdb,amyvmiwei/rocksdb,Andymic/rocksdb,ryneli/rocksdb,JohnPJenkins/rocksdb,hobinyoon/rocksdb,caijieming-baidu/rocksdb,NickCis/rocksdb,luckywhu/rocksdb,anagav/rocksdb,ylong/rocksdb,siddhartharay007/rocksdb,ryneli/rocksdb,wlqGit/rocksdb,sorphi/rocksdb,vmx/rocksdb,wlqGit/rocksdb,OverlordQ/rocksdb,flabby/rocksdb,OverlordQ/rocksdb,geraldoandradee/rocksdb,JackLian/rocksdb,tsheasha/rocksdb,ylong/rocksdb,Applied-Duality/rocksdb,fengshao0907/rocksdb,JackLian/rocksdb,tsheasha/rocksdb,temicai/rocksdb,biddyweb/rocksdb,dkorolev/rocksdb,tschottdorf/rocksdb,mbarbon/rocksdb,alihalabyah/rocksdb,vashstorm/rocksdb,mbarbon/rocksdb,ylong/rocksdb,skunkwerks/rocksdb,OverlordQ/rocksdb,tsheasha/rocksdb,skunkwerks/rocksdb,bbiao/rocksdb,makelivedotnet/rocksdb,jalexanderqed/rocksdb,alihalabyah/rocksdb,hobinyoon/rocksdb,hobinyoon/rocksdb,wat-ze-hex/rocksdb,tsheasha/rocksdb