fix error catching in cachedget
fix error catching in cachedget

avoids turning 404/400 into 500

Sage-Bionetworks/nbviewer,franblas/beaker-sharing-server,twosigma/beaker-sharing-server,franblas/beaker-sharing-server,AlfiyaZi/nbviewer,franblas/beaker-sharing-server,bollwyvl/nbviewer,twosigma/beaker-sharing-server,thomasyu888/nbviewer,iamjakob/nbviewer,franblas/beaker-sharing-server,christophelec/nbviewer,AlfiyaZi/nbviewer,Sage-Bionetworks/nbviewer,roxyboy/nbviewer,christophelec/nbviewer,twosigma/beaker-sharing-server,twosigma/beaker-sharing-server,iamjakob/nbviewer,imsparsh/nbviewer,thomasyu888/nbviewer,bollwyvl/nbviewer,Jay-Oh-eN/nbviewer,roxyboy/nbviewer,Jay-Oh-eN/nbviewer,imsparsh/nbviewer