Improve erorr message

Dependencies

• [2] FCTX433O Add buildStarted plugin hook
• [3] X4KYZJBQ Use latest nixUnstable
• [4] IE2PRAQU hydra-queue-runner: Send build notifications
• [*] 24BMQDZA Start of single-process hydra-queue-runner

Change contents

• replacement in src/hydra-queue-runner/hydra-queue-runner.cc at line 513
  [3.2339]→[3.2339:2464](∅→∅),[3.2464]→[2.952:990](∅→∅)
  − if (res != 0)

  − throw Error(format("hydra-build returned exit code %1% notifying about build %2%")

  − % res % item.id);
  [3.2339]

  [3.2505]
  + if (!statusOk(res))

  + throw Error("notification about build %d failed: %s", item.id, statusToString(res));