fix: include full stack traces in error logs

[derekderie]

Problem

Error logs across all job handlers only logged err.message, which drops the stack trace entirely. This makes it very hard to diagnose where errors originate.

Fix

• server/core/logger.js: add winston.format.errors({ stack: true })
  to the format chain, and update the printf format to append the stack
  when present.
• server/jobs/*.js (8 files): pass the full err object to
  WIKI.logger.error() instead of err.message.

[derekderie]

Experienced a lot of trouble in debugging #7978, had to create this PR in order to successfully uncover the root cause of the wiki crashes.