Improve error message when a depfile contains a bad path
Improve error message when a depfile contains a bad path

Debugging "ninja: error: empty path" is not fun, so make the error
message mention the depfile name.

juntalis/ninja,ninja-build/ninja,lizh06/ninja,vvvrrooomm/ninja,sxlin/dist_ninja,Maratyszcza/ninja-pypi,sxlin/dist_ninja,lizh06/ninja,sgraham/ninja,iwadon/ninja,vvvrrooomm/ninja,moroten/ninja,fuchsia-mirror/third_party-ninja,martine/ninja,iwadon/ninja,juntalis/ninja,fuchsia-mirror/third_party-ninja,Qix-/ninja,nicolasdespres/ninja,nicolasdespres/ninja,atetubou/ninja,vvvrrooomm/ninja,vvvrrooomm/ninja,bradking/ninja,moroten/ninja,nafest/ninja,ndsol/subninja,iwadon/ninja,sxlin/dist_ninja,sgraham/ninja,nafest/ninja,maruel/ninja,Maratyszcza/ninja-pypi,mydongistiny/ninja,AoD314/ninja,nico/ninja,ndsol/subninja,AoD314/ninja,Qix-/ninja,AoD314/ninja,martine/ninja,nico/ninja,Maratyszcza/ninja-pypi,moroten/ninja,ninja-build/ninja,sgraham/ninja,atetubou/ninja,moroten/ninja,mydongistiny/ninja,fuchsia-mirror/third_party-ninja,nico/ninja,juntalis/ninja,nicolasdespres/ninja,lizh06/ninja,martine/ninja,sxlin/dist_ninja,juntalis/ninja,atetubou/ninja,Qix-/ninja,maruel/ninja,maruel/ninja,ninja-build/ninja,AoD314/ninja,sgraham/ninja,sxlin/dist_ninja,iwadon/ninja,maruel/ninja,Maratyszcza/ninja-pypi,ninja-build/ninja,bradking/ninja,nico/ninja,nafest/ninja,bradking/ninja,lizh06/ninja,Qix-/ninja,sxlin/dist_ninja,ndsol/subninja,ndsol/subninja,mydongistiny/ninja,atetubou/ninja,nicolasdespres/ninja,fuchsia-mirror/third_party-ninja,mydongistiny/ninja,nafest/ninja,sxlin/dist_ninja,martine/ninja,bradking/ninja