Raise DockerBuildError on Docker build failures

[kailun-qin]

Description of the changes

Previously, build_docker_image() ignored the 'error' key in the Docker API stream, relying solely on get_docker_image() to catch build failures. This caused the script to incorrectly report success even if a build instruction failed, when a stale Graminized image existed.

This commit adds an explicit check for the 'error' key and introduces a DockerBuildError exception. All callers are updated to catch this exception and exit, ensuring build failures are correctly reported.

How to test this PR?

Manual testing: run gsc build w/ the python:bullseye example from our doc.

Before this fix:

$ docker images
REPOSITORY   TAG                 IMAGE ID       CREATED        SIZE
gsc-python   bullseye-unsigned   e647502b2b77   3 hours ago    1.04GB

$ ./gsc build --insecure-args python:bullseye test/generic.manifest
...
Err:5 http://deb.debian.org/debian bullseye-backports Release
  404  Not Found [IP: 10.224.224.80 911]
...
Reading package lists...

E: The repository 'http://deb.debian.org/debian bullseye-backports Release' does not have a Release file.

Successfully built an unsigned graminized Docker image `gsc-python:bullseye-unsigned` from original application image `python:bullseye`.

After this fix:

E: The repository 'http://deb.debian.org/debian bullseye-backports Release' does not have a Release file.

Docker build failed: The command '/bin/sh -c echo 'deb http://deb.debian.org/debian bullseye-backports main' > /etc/apt/sources.list.d/backports.list     && env DEBIAN_FRONTEND=noninteractive apt-get update     && env DEBIAN_FRONTEND=noninteractive apt-get install -y -t bullseye-backports linux-libc-dev' returned a non-zero code: 100

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This change is 

[kailun-qin]

Please note that this works fine under a clean setup. Not sure if we consider this as a false positive -- I'm raising it here anyway.

@kailun-qin made 1 comment.
Reviewable status: 0 of 1 files reviewed, all discussions resolved, not enough approvals from maintainers (2 more required), not enough approvals from different teams (1 more required, approved so far: Intel).