container: Gracefully report mount errors in the child process.

container: Gracefully report mount errors in the child process.

Fixes <http://bugs.gnu.org/23306>.

* gnu/build/linux-container.scm (run-container): Use 'socketpair'
instead of 'pipe'.  Rename 'in' to 'child' and 'out' to 'parent'.  Send
a 'ready message or an exception argument list from the child to the
parent; adjust the parent accordingly.
* tests/containers.scm ("call-with-container, mnt namespace, wrong bind
mount"): New test.
* tests/guix-environment-container.sh: Add test with
--expose=/does-not-exist.