Changing the message when no tasks selected

[aspiringmind-code]

The failure description was misleading as it would give the same even when there are no tasks to select.

[cmsdmwmbot]

Jenkins results:

• Python3 Pylint check: succeeded
  • 49 comments to review
• Pycodestyle check: succeeded
  • 58 comments to review

Details at https://cmssdt.cern.ch/dmwm-jenkins/view/All/job/DMWM-CRABServer-PR-test/2201/artifact/artifacts/PullRequestReport.html

[belforte]

THANKS