Add FailedBuild status for faster error detection

[hoverduck]

Currently the canary tests that run against dserve don't have any way to detect when an image fails to build, so they need to wait for the full 10-minute timeout to report failure back to the PR. It would be helpful to have the /status endpoint register that there was a failure, and even provide the error message (i.e Encountered error when building image vs Encountered error while git checking out, tarballing, or handing to docker) for display on the CircleCI output

[samouri]

This shouldn't be too bad. I don't think we currently make any changes to the fs to mark a branch as failed. My guess is that implementing this intelligently could be ~20 LOC