cry: propagate command failure

[lisanna-dettwyler]

Closes #953

[lisanna-dettwyler]

The original issue was a spurious crash, likely a problem with the VM. I reproduced the masking behavior by passing an invalid target to cry build and observed bash continuing past it: https://github.com/GaloisInc/cryptol/runs/1408891733#step:8:65

The fix here is to make sure that the cry script correctly propagates build failures. I tested this fix with my original contrived reproducer and the build correctly fails at that step (after trying it 3 times): https://github.com/GaloisInc/cryptol/runs/1408908452#step:8:67