For us it would be nicer if the Carthage step would not fail if the Cartfile could not be found.
You could place a message in the logs about what happened and let the step succeed. Just treat it as if there are no dependencies.
This would make adding Carthage much easier.
I have a long running branch for the migration from Cocoapods to Carthage. Therefore I often need to add and remove the Carthage step from my workflow. It would be nicer if the step just succeeds if there is no Cartfile instead of fail the whole integration. This is ok since if a dependency is not found the compiler complains anyway and since you could print a log message the user can figure out whats wrong.