Add workaround and WARNING for pulp issue #6811 (#808)
Add workaround and WARNING for pulp issue #6811 (#808)

Until the Pulp developers decide how to handle #6811, this change will
work around the issue in most circumstances.

I also added a warning for when a sync is attempted but nothing changes,
which is what #6811 looks like when it occurs.
ros-infrastructure/ros_buildfarm,ros-infrastructure/ros_buildfarm,ros-infrastructure/ros_buildfarm