Show output on execution failure if not expected

[szpak]

Doing some refactoring I often have integration tests with failing tasks. In many cases the error message (task 'xxx' failed) is not enough to get know what happened. What's more I cannot simply add println result.standardoutput as the test execution is aborted (I need to switch to runTasksWithFailure()).
It would be handy to have a switch printOutputOnFailure (disabled by default) to print stdout (and stderr?) on failure when runTasksSuccessfully() was used.