hydra-build: Give a nicer error message if the derivation is gone

Dependencies

• [2] PMNWRTGJ Add multiple output support
• [*] 7YBYT2LQ
• [*] 37R34XJO * Negative caching: don't perform a build if a dependency already

Change contents

• edit in src/script/hydra-build at line 117
  [5.3434]

  [2.16072]
  + if (!isValidPath($drvPath)) {

  + $buildStatus = 3;

  + $errormsg = "derivation was garbage-collected prior to build";

  + goto done;

  + }

  +