Improve postStartCommand output/error handling

[Pwuts]

Currently, when a postStartCommand fails, the log doesn't contain any information about why the command failed, making it hell to debug. Would be nice if this could be fixed.

Example:

2023-03-28 16:01:47.780Z: echo -n 'Waiting for MySQL to come online'; bash -c 'until [[ `docker ps --format="{{ .Names }}" 2>/dev/null | grep mysql` ]]; do echo -n .; sleep 0.5; done; export MYSQL_CONTAINER=`docker ps --format="{{ .Names }}" | grep mysql`;
2023-03-28 16:02:20.762Z: postStartCommand failed with exit code 1. Skipping any further user-provided commands.

2023-03-28 16:02:20.775Z: Error: Command failed: /bin/sh -c echo -n 'Waiting for MySQL to come online'; bash -c 'until [[ `docker ps --format="{{ .Names }}" 2>/dev/null | grep mysql` ]]; do echo -n .; sleep 0.5; done; export MYSQL_CONTAINER=`docker ps --format="{{ .Names }}" | grep mysql`;
2023-03-28 16:02:20.780Z:     at wN (/usr/lib/node_modules/@devcontainers/cli/dist/spec-node/devContainersSpecCLI.js:1708:130)
2023-03-28 16:02:20.844Z:     at processTicksAndRejections (internal/process/task_queues.js:95:5)
2023-03-28 16:02:20.850Z:     at async Tg (/usr/lib/node_modules/@devcontainers/cli/dist/spec-node/devContainersSpecCLI.js:1700:4370)
2023-03-28 16:02:20.870Z:     at async DN (/usr/lib/node_modules/@devcontainers/cli/dist/spec-node/devContainersSpecCLI.js:1700:3895)
2023-03-28 16:02:20.875Z:     at async Pg (/usr/lib/node_modules/@devcontainers/cli/dist/spec-node/devContainersSpecCLI.js:1700:3097)
2023-03-28 16:02:20.880Z: devcontainer process exited with exit code 1
2023-03-28 16:02:20.886Z:     at async Ka (/usr/lib/node_modules/@devcontainers/cli/dist/spec-node/devContainersSpecCLI.js:1700:2393)
2023-03-28 16:02:20.894Z:     at async uie (/usr/lib/node_modules/@devcontainers/cli/dist/spec-node/devContainersSpecCLI.js:1869:3362)
2023-03-28 16:02:20.911Z:     at async kie (/usr/lib/node_modules/@devcontainers/cli/dist/spec-node/devContainersSpecCLI.js:1936:2807)
2023-03-28 16:02:20.919Z:     at async zf (/usr/lib/node_modules/@devcontainers/cli/dist/spec-node/devContainersSpecCLI.js:1936:3759)
2023-03-28 16:02:20.925Z:     at async uoe (/usr/lib/node_modules/@devcontainers/cli/dist/spec-node/devContainersSpecCLI.js:2060:17560)
2023-03-28 16:02:20.942Z: {"outcome":"error","message":"Command failed: /bin/sh -c echo -n 'Waiting for MySQL to come online'; bash -c 'until [[ `docker ps --format=\"{{ .Names }}\" 2>/dev/null | grep mysql` ]]; do echo -n .; sleep 0.5; done; export MYSQL_CONTAINER=`docker ps --format=\"{{ .Names }}\" | grep mysql`;","description":"The postStartCommand in the devcontainer.json failed.","containerId":"9715fe9bed2786835d01b2eddbdb40a555a8c1e8679c6219add72515812be1a2"}

Similarly, when the command works as intended, it is shown in the terminal of my Codespace, but the output is not shown there, even though it ends up in the creationlog after the command finishes.