Just log exceptions instead of spamming them in JobRunner

* Even the CLI script already shows the error=X snippet.
  Logging the error should be enough, and avoids showing
  output when all we want is the JSON.

Change-Id: Iade412ea61cf427865d841ecab5498e4fcdb7e13
(cherry picked from commit e0ad91f058e3dd132a18f6b99cb8216996637db8)