raise error for Graph/XGraph on get_edge for nonexistent edge to match XGraph/XDiGraph
raise error for Graph/XGraph on get_edge for nonexistent edge
to match XGraph/XDiGraph

--HG--
extra : convert_revision : svn%3A3ed01bd8-26fb-0310-9e4c-ca1a4053419f/networkx/trunk%40431

jtorrents/networkx,debsankha/networkx,ionanrozenfeld/networkx,RMKD/networkx,dmoliveira/networkx,dhimmel/networkx,aureooms/networkx,debsankha/networkx,dhimmel/networkx,kai5263499/networkx,goulu/networkx,ltiao/networkx,cmtm/networkx,debsankha/networkx,kernc/networkx,bzero/networkx,JamesClough/networkx,jtorrents/networkx,dmoliveira/networkx,yashu-seth/networkx,blublud/networkx,sharifulgeo/networkx,ionanrozenfeld/networkx,tmilicic/networkx,harlowja/networkx,kernc/networkx,chrisnatali/networkx,SanketDG/networkx,bzero/networkx,bzero/networkx,blublud/networkx,sharifulgeo/networkx,jni/networkx,dhimmel/networkx,ionanrozenfeld/networkx,jni/networkx,ghdk/networkx,wasade/networkx,jcurbelo/networkx,jakevdp/networkx,jakevdp/networkx,jfinkels/networkx,Sixshaman/networkx,aureooms/networkx,nathania/networkx,sharifulgeo/networkx,chrisnatali/networkx,farhaanbukhsh/networkx,blublud/networkx,RMKD/networkx,jakevdp/networkx,NvanAdrichem/networkx,RMKD/networkx,harlowja/networkx,aureooms/networkx,dmoliveira/networkx,harlowja/networkx,farhaanbukhsh/networkx,beni55/networkx,michaelpacer/networkx,kai5263499/networkx,nathania/networkx,andnovar/networkx,kai5263499/networkx,farhaanbukhsh/networkx,nathania/networkx,chrisnatali/networkx,kernc/networkx,OrkoHunter/networkx,ghdk/networkx,ghdk/networkx,jni/networkx