Return error if can't register digest handler
Return error if can't register digest handler

This was caught by go vet.

vegasbrianc/cadvisor,difro/cadvisor,dalanlan/cadvisor,Clever/cadvisor,f0/cadvisor,bowlofstew/cadvisor,dchen1107/cadvisor,IFTTT/cadvisor,IFTTT/cadvisor,pwittrock/cadvisor,mangalaman93/cadvisor,BrickXu/cadvisor,aivarasko/cadvisor,jimonreal/cadvisor,jmaitrehenry/cadvisor,timstclair/cadvisor,whosthatknocking/cadvisor,CrazyJvm/cadvisor,difro/cadvisor,whosthatknocking/cadvisor,derwolfe/cadvisor,yolkov/cadvisor,datawisesystems/cadvisor,bpradipt/cadvisor,rajthilakmca/cadvisor,f0/cadvisor,vishh/cadvisor,takecy/cadvisor,mr-justin/cadvisor,Snorch/cadvisor,jzaefferer/cadvisor,f0/cadvisor,BrickXu/cadvisor,guoshimin/cadvisor,ainsleyc/cadvisor,hzy001/cadvisor,morpheyesh/cadvisor,mesosphere/cadvisor,timstclair/cadvisor,hw-qiaolei/cadvisor,guoshimin/cadvisor,derekwaynecarr/cadvisor,erSitzt/cadvisor,ncdc/cadvisor,derwolfe/cadvisor,kmicheal/cadvisor,erSitzt/cadvisor,erSitzt/cadvisor,bpradipt/cadvisor,ChengTiesheng/cadvisor,yifan-gu/cadvisor,ChengTiesheng/cadvisor,f0/cadvisor,ChengTiesheng/cadvisor,datawisesystems/cadvisor,hacpai/cadvisor,bing6/cadvisor,mesosphere/cadvisor,CrazyJvm/cadvisor,shilpamayanna/cadvisor,Snorch/cadvisor,ainsleyc/cadvisor,jhjeong-kr/cadvisor,guoshimin/cadvisor,jzaefferer/cadvisor,dchen1107/cadvisor,Snorch/cadvisor,ncdc/cadvisor,bowlofstew/cadvisor,yifan-gu/cadvisor,ncdc/cadvisor,jmaitrehenry/cadvisor,megamsys/cadvisor,shilpamayanna/cadvisor,erSitzt/cadvisor,kmicheal/cadvisor,jimmidyson/cadvisor,jhjeong-kr/cadvisor,dalanlan/cadvisor,normanjoyner/cadvisor,jimmidyson/cadvisor,vishh/cadvisor,jimonreal/cadvisor,whosthatknocking/cadvisor,atyenoria/cadvisor,dchen1107/cadvisor,afein/cadvisor,erSitzt/cadvisor,feiskyer/cadvisor,derekwaynecarr/cadvisor,jimonreal/cadvisor,normanjoyner/cadvisor,megamsys/cadvisor,vishh/cadvisor,f0/cadvisor,jhjeong-kr/cadvisor,atyenoria/cadvisor,CrazyJvm/cadvisor,bing6/cadvisor,yolkov/cadvisor,atyenoria/cadvisor,rajthilakmca/cadvisor,yolkov/cadvisor,c-st/cadvisor,mesosphere/cadvisor,ainsleyc/cadvisor,morpheyesh/cadvisor,derwolfe/cadvisor,hacpai/cadvisor,Clever/cadvisor,anarcher/cadvisor,linux-on-ibm-z/cadvisor,dchen1107/cadvisor,afein/cadvisor,wulonghui/cadvisor,derekwaynecarr/cadvisor,takecy/cadvisor,ncdc/cadvisor,kmicheal/cadvisor,c-st/cadvisor,derekwaynecarr/cadvisor,anarcher/cadvisor,jimmidyson/cadvisor,jhjeong-kr/cadvisor,mr-justin/cadvisor,jmaitrehenry/cadvisor,ainsleyc/cadvisor,ncdc/cadvisor,BrickXu/cadvisor,vegasbrianc/cadvisor,dchen1107/cadvisor,wulonghui/cadvisor,takecy/cadvisor,IFTTT/cadvisor,bowlofstew/cadvisor,pwittrock/cadvisor,hw-qiaolei/cadvisor,vegasbrianc/cadvisor,aivarasko/cadvisor,c-st/cadvisor,carmark/cadvisor,hacpai/cadvisor,yifan-gu/cadvisor,mr-justin/cadvisor,linux-on-ibm-z/cadvisor,BrickXu/cadvisor,jzaefferer/cadvisor,hzy001/cadvisor,anarcher/cadvisor,bpradipt/cadvisor,IFTTT/cadvisor,bing6/cadvisor,IFTTT/cadvisor,pwittrock/cadvisor,hw-qiaolei/cadvisor,wulonghui/cadvisor,derekwaynecarr/cadvisor,mangalaman93/cadvisor,feiskyer/cadvisor,timstclair/cadvisor,dalanlan/cadvisor,BrickXu/cadvisor,linux-on-ibm-z/cadvisor,aivarasko/cadvisor,hzy001/cadvisor,jhjeong-kr/cadvisor,carmark/cadvisor,difro/cadvisor,feiskyer/cadvisor,rajthilakmca/cadvisor,Clever/cadvisor,mangalaman93/cadvisor