Tidy bind_mount_test
Tidy bind_mount_test

- Remove unnecessary test of container creation error case
- Add explicit error checking for all actual container creations

Hopefully this will result in better error messages when the flake
[#108236872] next occurs in the wild.

Signed-off-by: Julian Friedman <f594f6672fb8d56d48c999a0f5f9e47ae2445e2b@uk.ibm.com>

cloudfoundry-incubator/garden-linux,cloudfoundry-incubator/garden-linux,cloudfoundry-incubator/garden-linux