Fix uninitialized value warning

Dependencies

• [2] SGRUPSLK Only send email notification of evaluation error when the evaluation error has changed. Fixes #121.
• [3] H7CNGK4O * Log evaluation errors etc. in the DB.
• [4] LZO3C2KI * Hack around those SQLite timeouts: just retry the transaction.
• [5] NLJJZVHO * Use ->update({...}) properly.
• [6] WQXF2T3D hydra-evaluator: Don't require $HYDRA_CONFIG
• [7] ZDENAYQI * email notification of evaluation errors to project owner (if desired)
• [8] SS4TZXNU Distinguish between permanent evaluation errors and transient input errors
• [*] N22GPKYT * Put info about logs / build products in the DB.

Change contents

• replacement in src/script/hydra-evaluator at line 52
  [3.3184]→[2.41:76](∅→∅)
  − if ($errorMsg ne $prevError) {
  [3.3184]

  [2.76]
  + if (defined $errorMsg && $errorMsg ne ($prevError // "")) {