Add FailedBuild status for faster error detection

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

Start at dserve's /status endpoint and trace how canary tests report image-build and other failures to CircleCI. Confirm how the existing timeout path distinguishes checkout, tarballing, and Docker errors; done means the endpoint reports FailedBuild promptly with the relevant error message visible in the PR's CircleCI output.

Written by the indexing model from the issue text.