Check if test_branch exists in at least one repo among the ones in repos file

[adityapande-1995]

Currently, if the supplied test_branch is not found in any of the repos in the .repos file, the workflow will just skip it and run CI on the default branches. There should be an error, or at least a warning saying that none of the repos contain the branch, as discussed offline with @clalancette

Closes https://github.com/ros2/ci/issues/620

Signed-off-by: Aditya [email protected]