Respect --keep-going when a substituter fails.
Respect --keep-going when a substituter fails.

Fixes #977

pikajude/nix,layus/nix,ysangkok/nix,NixOS/nix,peti/nix,bennofs/nix,rycee/nix,dtzWill/nix,layus/nix,dtzWill/nix,shlevy/nix,zimbatm/nix,shlevy/nix,ehmry/nix,dezgeg/nix,NixOS/nix,pikajude/nix,NixOS/nix,ehmry/nix,bennofs/nix,ehmry/nix,peti/nix,rycee/nix,shlevy/nix,peti/nix,zimbatm/nix,layus/nix,peti/nix,NixOS/nix,bennofs/nix,pikajude/nix,NixOS/nix,dezgeg/nix,dtzWill/nix,dtzWill/nix,rycee/nix,vcunat/nix,layus/nix,bennofs/nix,zimbatm/nix,shlevy/nix,ehmry/nix,dtzWill/nix,layus/nix,dezgeg/nix,vcunat/nix,vcunat/nix,peti/nix,vcunat/nix,rycee/nix,rycee/nix,ysangkok/nix,shlevy/nix,ehmry/nix,dezgeg/nix,ehmry/nix,ysangkok/nix,zimbatm/nix,dezgeg/nix