hydra-queue-runner: better error message if nix-store can not be started

Dependencies

• [2] 4VYY2ADP Allow the machines file to specify host public keys
• [3] 7VQ4ALFY Update "make check" for the new queue runner
• [4] 5AIYUMTB Basic remote building
• [5] XCDTFZUY hydra-queue-runner: Fix build

Change contents

• edit in src/hydra-queue-runner/build-remote.cc at line 32
  [3.490]

  [3.490]
  + string pgmName;
• replacement in src/hydra-queue-runner/build-remote.cc at line 51
  [3.297]→[2.117:162](∅→∅)
  − if (machine->sshName == "localhost")
  [3.297]

  [3.333]
  + if (machine->sshName == "localhost") {

  + pgmName = "nix-store";
• edit in src/hydra-queue-runner/build-remote.cc at line 54
  [3.389]

  [3.389]
  + }
• edit in src/hydra-queue-runner/build-remote.cc at line 56
  [3.404]

  [2.163]
  + pgmName = "ssh";
• replacement in src/hydra-queue-runner/build-remote.cc at line 73
  [3.1100]→[3.1100:1144](∅→∅)
  − throw SysError("cannot start ssh");
  [3.1100]

  [3.1144]
  + throw SysError("cannot start %s", pgmName);