Improve error output

In #184, the output from the Azure DevOps task was not especially helpful in identifying the underlying issue:

```
About to run devcontainer exec --workspace-folder /home/vsts/work/1/s bash -c echo "hi"
### ERROR: Dev container exec: Command failed: docker ps -q -a --filter label=devcontainer.local_folder=/home/vsts/work/1/s (exit code: undefined)
An error occurred running a command in the container.
##[error]Command failed: docker ps -q -a --filter label=devcontainer.local_folder=/home/vsts/work/1/s
```

It turned out to be a command run as part of the postCreateCommand step that broke the docker permissions, but there was nothing output in the logs that gave any indication of this.

It would be good to see whether there is an exit code and or output from the `docker` command that could be output to aid debugging in future.