def check_dependencies(specs)
      missing = check_app_requirement(specs)

      present = exclude_specs(installed, specs)
      future = present + specs

      check_all_dependencies(specs, future, missing)
    end
def check_dependencies(specs)
      missing = check_app_requirement(specs)

      present = exclude_specs(installed, specs)
      future = present + specs

      check_all_dependencies(specs, future, missing)
    end
Check if all wagon dependencies of the given gem specifications
 are met by the installed wagons.
 Returns nil if everything is fine or a string with error messages.