Return :warn instead of :fail on BundleCheck

When rebasing, returning :fail will abort rebase and the repository ends
up with a detached HEAD. Rebase becomes impossible unless you install
the needed gems first.

Change-Id: I6bc22934554dc4b422398082ccf076de97fd0251
Reviewed-on: http://gerrit.causes.com/35952
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>