Add a quota for doc job generation and error with a clear message if it's too big

[tfoote]

Instead of just uploading very large files to our hosting service.

We could set a reasonable limit to make sure that people don't abuse the system on purpose or by accident.

We're using up a lot of disk space at the moment. Related ticket for reducing output size: https://github.com/ros-infrastructure/rosdoc2/issues/56