Unittest - Include "task diag" output on error

* If any command passed to runSuccess or runError returns an unexpected
exit code, the output of 'task diag' will be added to stdout/stderr.
Merge pull request #6 in TM/task from ~UNODE/task:2.4.0 to 2.4.0

* commit '4ae56838e4a9ec2b8892295d1ea9a1f89e4b7f6c':
  Unittest - Include "task diag" output on error