People keep getting confused when the root partition runs out of space during build_image. In many cases they will think that the builder itself is running out of space and misdiagnose the problem:
Issue 630426 (associated postmortem https://docs.google.com/a/google.com/document/d/1ooanmwstBOvhjyVbifaaUEBmyazlzPGYQEyKH9JjwmQ/edit?usp=sharing)
Issue 775061
 Issue 777874 

We should try to catch that error and add a very detailed error message explaining that it's not the builder running out of space, it's the image.