Improve error handling when mounting the bottlerocket source code fails.

[jmt-lab]

A customer attempted to build bottlerocket variants inside of a docker container with the docker daemon socket shared between the host and the container. The build failed when buildsys orchestrated docker build to mount the sources into the build context because of the directory going from container volume -> host -> build-context. (bottlerocket-os/bottlerocket#4100)

However, the error message is rather unhelpful and doesn't give any actionable feedback. We should investigate if we can handle this case more gracefully.

2024-07-23 23:46:20 [ERROR] the directory /tmp/sources doesn't contain a Cargo.toml file
Error while executing command, exit code: 1
Error: Command was unsuccessful, exit code 105
Error while executing command, exit code: 1