CDAP-821 Improve CLI error messages
Improve log messages and comments.
Merge pull request #757 from caskdata/feature/fix-error-message

Improve log messages and comments.
CDAP-838 Twill runnables should exit when any of the chained services complete
Merge pull request #756 from caskdata/feature/cdap-838_tx-abort

CDAP-838 Twill runnables should exit when any of the chained services complete
Merge pull request #754 from caskdata/feature/CDAP-821-cli-error-message

CDAP-821 Improve CLI error messages
Spark Integration: Docs for spark 2.6.0 features
Addressed PR comments