Improve email notifications status messages

Dependencies

• [2] 5EQYVRWE Add a plugin mechanism
• [3] QDJRQIFF If multiple dependent builds fail, send a single email
• [4] LDICTHWY Use Template::Toolkit to generate email messages

Change contents

• replacement in src/lib/Hydra/Plugin/EmailNotification.pm at line 24
  [2.482]→[2.482:547](∅→∅)
  − when (1) { $status = "Failed with non-zero exit code"; }
  [2.482]

  [2.547]
  + when (1) { $status = "Failed"; }
• edit in src/lib/Hydra/Plugin/EmailNotification.pm at line 27
  [2.643]

  [2.643]
  + when (6) { $status = "Failed with output"; }